diff --git a/STATUS.md b/STATUS.md
index 8a2b0fd..c825f33 100644
--- a/STATUS.md
+++ b/STATUS.md
@@ -7,10 +7,11 @@ Living progress tracker. Update at the end of each task. The canonical brief is
 - **01 — Repo init & tooling** (commit `b89690a`). pnpm 10 + TS strict + ESLint flat + Prettier. All acceptance criteria green.
 - **02 — Next.js app skeleton**. Next.js 15 + React 19 + Tailwind v4 + shadcn/ui (button, card, input, dialog). `pnpm dev` serves placeholder, `pnpm build` produces `.next/standalone/`, `pnpm lint` clean. Added `.npmrc` with `node-linker=hoisted` for Windows symlink compatibility.
 - **03 — Drizzle + Postgres setup**. drizzle-orm + postgres driver + drizzle-kit wired up. `src/modules/_core/schema.ts` declares `users`, `households`, `household_members`. `docker-compose.dev.yaml` starts Postgres 16. `drizzle/0000_silent_magma.sql` generated and applied. `tsc --noEmit` passes.
+- **04 — Module loader & registry**. `src/modules/_core/module.ts` types (`ModuleManifest`, `EntityTypeRegistration`, `DashboardWidget`, etc.), `registry.ts` singleton with `registerModule`/`getRegistry`/`getEntityType`/`getWidget`, barrel `_core/index.ts`. Stub manifests for `calendar`, `lists`, `notes`. `src/modules/index.ts` loader. Root layout imports loader; `AppNav` reads registry for nav links. `/debug/registry` dumps full registry JSON (dev only). Uses zod v4 + built-in `z.toJSONSchema()`. `tsc --noEmit`, `pnpm build`, `pnpm lint` all clean.
 
 ## Next up
 
-- **04 — Module loader** ([brief](docs/tasks/04-module-loader.md)). Phase 1 is sequential through 08.
+- **05 — Compose + Caddy** ([brief](docs/tasks/05-compose-caddy.md)).
 
 ## Phase 1 remaining
 
diff --git a/package.json b/package.json
index 5dd3c94..cd9156c 100644
--- a/package.json
+++ b/package.json
@@ -48,6 +48,8 @@
     "react-dom": "^19.2.5",
     "shadcn": "^4.7.0",
     "tailwind-merge": "^3.5.0",
-    "tw-animate-css": "^1.4.0"
+    "tw-animate-css": "^1.4.0",
+    "zod": "^4.4.3",
+    "zod-to-json-schema": "^3.25.2"
   }
 }
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index d3baa20..d347444 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -44,6 +44,12 @@ importers:
       tw-animate-css:
         specifier: ^1.4.0
         version: 1.4.0
+      zod:
+        specifier: ^4.4.3
+        version: 4.4.3
+      zod-to-json-schema:
+        specifier: ^3.25.2
+        version: 3.25.2(zod@4.4.3)
     devDependencies:
       '@eslint/eslintrc':
         specifier: ^3.3.5
@@ -7142,6 +7148,10 @@ snapshots:
     dependencies:
       zod: 3.25.76
 
+  zod-to-json-schema@3.25.2(zod@4.4.3):
+    dependencies:
+      zod: 4.4.3
+
   zod-validation-error@4.0.2(zod@4.4.3):
     dependencies:
       zod: 4.4.3
diff --git a/src/app/debug/registry/page.tsx b/src/app/debug/registry/page.tsx
new file mode 100644
index 0000000..98cc0cf
--- /dev/null
+++ b/src/app/debug/registry/page.tsx
@@ -0,0 +1,51 @@
+import { notFound } from "next/navigation";
+import { z } from "zod";
+import { getRegistry } from "@/modules/_core/registry";
+import "@/modules";
+
+export default function RegistryDebugPage() {
+  if (process.env.NODE_ENV !== "development") {
+    notFound();
+  }
+
+  const { modules } = getRegistry();
+
+  const output = {
+    modules: modules.map((m) => ({
+      id: m.id,
+      name: m.name,
+      nav: m.nav ?? null,
+      entities: m.entities.map((e) => ({
+        type: e.type,
+        label: e.label,
+        hasShare: !!e.share,
+        hasReminder: !!e.reminder,
+        hasSearch: !!e.search,
+      })),
+      dashboardWidgets: (m.dashboardWidgets ?? []).map((w) => ({
+        id: w.id,
+        title: w.title,
+        description: w.description,
+        category: w.category ?? null,
+        defaultSize: w.defaultSize,
+        defaultPriority: w.defaultPriority,
+        configSchema: z.toJSONSchema(w.configSchema),
+        defaultConfig: w.defaultConfig,
+      })),
+      quickAdds: (m.quickAdds ?? []).map((q) => ({
+        id: q.id,
+        label: q.label,
+        icon: q.icon ?? null,
+      })),
+    })),
+  };
+
+  return (
+    <div className="p-8">
+      <h1 className="text-2xl font-bold mb-4">Module Registry (dev only)</h1>
+      <pre className="text-xs bg-muted rounded p-4 overflow-auto">
+        {JSON.stringify(output, null, 2)}
+      </pre>
+    </div>
+  );
+}
diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index 05472e2..5273a42 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -2,8 +2,10 @@ import type { Metadata } from "next";
 import "./globals.css";
 import { Geist } from "next/font/google";
 import { cn } from "@/lib/utils";
+import { AppNav } from "@/components/app-nav";
+import "@/modules"; // registers all module manifests
 
-const geist = Geist({subsets:['latin'],variable:'--font-sans'});
+const geist = Geist({ subsets: ["latin"], variable: "--font-sans" });
 
 export const metadata: Metadata = {
   title: "famapp",
@@ -17,7 +19,10 @@ export default function RootLayout({
 }) {
   return (
     <html lang="en" className={cn("font-sans", geist.variable)}>
-      <body>{children}</body>
+      <body className="min-h-screen">
+        <AppNav />
+        <main>{children}</main>
+      </body>
     </html>
   );
 }
diff --git a/src/components/app-nav.tsx b/src/components/app-nav.tsx
new file mode 100644
index 0000000..df2fbae
--- /dev/null
+++ b/src/components/app-nav.tsx
@@ -0,0 +1,24 @@
+import Link from "next/link";
+import { getRegistry } from "@/modules/_core/registry";
+
+export function AppNav() {
+  const { modules } = getRegistry();
+  const navItems = modules.flatMap((m) => (m.nav ? [m.nav] : []));
+
+  return (
+    <nav className="border-b px-4 py-3 flex items-center gap-6">
+      <Link href="/" className="font-semibold text-sm">
+        famapp
+      </Link>
+      {navItems.map((item) => (
+        <Link
+          key={item.href}
+          href={item.href}
+          className="text-sm text-muted-foreground hover:text-foreground transition-colors"
+        >
+          {item.label}
+        </Link>
+      ))}
+    </nav>
+  );
+}
diff --git a/src/modules/_core/index.ts b/src/modules/_core/index.ts
new file mode 100644
index 0000000..fd530eb
--- /dev/null
+++ b/src/modules/_core/index.ts
@@ -0,0 +1,12 @@
+export type {
+  ModuleManifest,
+  EntityTypeRegistration,
+  DashboardWidget,
+  QuickAddAction,
+  WidgetContext,
+  ShareCapabilities,
+  ReminderCapabilities,
+  SearchAdapter,
+  SearchResult,
+} from "./module";
+export { registerModule, getRegistry, getEntityType, getWidget } from "./registry";
diff --git a/src/modules/_core/module.ts b/src/modules/_core/module.ts
new file mode 100644
index 0000000..e35f3b2
--- /dev/null
+++ b/src/modules/_core/module.ts
@@ -0,0 +1,68 @@
+import type { ReactNode } from "react";
+import type { ZodType } from "zod";
+
+export type ShareCapabilities = {
+  canShare: boolean;
+  defaultCapabilities?: string[];
+};
+
+export type ReminderCapabilities = {
+  canRemind: boolean;
+};
+
+export type SearchResult = {
+  id: string;
+  title: string;
+  url: string;
+  excerpt?: string;
+};
+
+export type SearchAdapter = {
+  search: (query: string, householdId: string) => Promise<SearchResult[]>;
+};
+
+export type WidgetContext = {
+  userId: string;
+  householdId: string;
+};
+
+export type DashboardWidget = {
+  id: string;
+  title: string;
+  description: string;
+  category?: string;
+  defaultSize: { w: number; h: number };
+  minSize?: { w: number; h: number };
+  maxSize?: { w: number; h: number };
+  defaultPriority: number;
+  configSchema: ZodType;
+  defaultConfig: unknown;
+  resolveConfigOptions?: (ctx: WidgetContext) => Promise<unknown>;
+  render: (props: { config: unknown; ctx: WidgetContext }) => ReactNode;
+};
+
+export type QuickAddAction = {
+  id: string;
+  label: string;
+  icon?: string;
+  action: (ctx: WidgetContext) => void | Promise<void>;
+};
+
+export type EntityTypeRegistration = {
+  type: string;
+  label: { singular: string; plural: string };
+  share?: ShareCapabilities;
+  reminder?: ReminderCapabilities;
+  search?: SearchAdapter;
+  resolveUrl: (id: string) => string;
+  loadForShare?: (id: string) => Promise<unknown>;
+};
+
+export type ModuleManifest = {
+  id: string;
+  name: string;
+  nav?: { href: string; label: string; icon?: string };
+  entities: EntityTypeRegistration[];
+  dashboardWidgets?: DashboardWidget[];
+  quickAdds?: QuickAddAction[];
+};
diff --git a/src/modules/_core/registry.ts b/src/modules/_core/registry.ts
new file mode 100644
index 0000000..7cb98a3
--- /dev/null
+++ b/src/modules/_core/registry.ts
@@ -0,0 +1,31 @@
+import type { ModuleManifest, EntityTypeRegistration, DashboardWidget } from "./module";
+
+const modules = new Map<string, ModuleManifest>();
+const entityTypes = new Map<string, EntityTypeRegistration>();
+const widgets = new Map<string, DashboardWidget>();
+
+export function registerModule(manifest: ModuleManifest): void {
+  modules.set(manifest.id, manifest);
+  for (const entity of manifest.entities) {
+    entityTypes.set(entity.type, entity);
+  }
+  for (const widget of manifest.dashboardWidgets ?? []) {
+    widgets.set(widget.id, widget);
+  }
+}
+
+export function getRegistry() {
+  return Object.freeze({
+    modules: Object.freeze([...modules.values()]),
+    entityTypes: Object.freeze([...entityTypes.values()]),
+    widgets: Object.freeze([...widgets.values()]),
+  });
+}
+
+export function getEntityType(type: string): EntityTypeRegistration | undefined {
+  return entityTypes.get(type);
+}
+
+export function getWidget(id: string): DashboardWidget | undefined {
+  return widgets.get(id);
+}
diff --git a/src/modules/calendar/manifest.ts b/src/modules/calendar/manifest.ts
new file mode 100644
index 0000000..abf7310
--- /dev/null
+++ b/src/modules/calendar/manifest.ts
@@ -0,0 +1,23 @@
+import type { ModuleManifest } from "../_core/module";
+
+const manifest: ModuleManifest = {
+  id: "calendar",
+  name: "Calendar",
+  nav: { href: "/calendar", label: "Calendar", icon: "calendar" },
+  entities: [
+    {
+      type: "calendar.calendar",
+      label: { singular: "Calendar", plural: "Calendars" },
+      resolveUrl: (id) => `/calendar?id=${id}`,
+    },
+    {
+      type: "calendar.event",
+      label: { singular: "Event", plural: "Events" },
+      resolveUrl: (id) => `/calendar/events/${id}`,
+    },
+  ],
+  dashboardWidgets: [],
+  quickAdds: [],
+};
+
+export default manifest;
diff --git a/src/modules/index.ts b/src/modules/index.ts
new file mode 100644
index 0000000..4ea8d1b
--- /dev/null
+++ b/src/modules/index.ts
@@ -0,0 +1,8 @@
+import { registerModule } from "./_core/registry";
+import calendarManifest from "./calendar/manifest";
+import listsManifest from "./lists/manifest";
+import notesManifest from "./notes/manifest";
+
+registerModule(calendarManifest);
+registerModule(listsManifest);
+registerModule(notesManifest);
diff --git a/src/modules/lists/manifest.ts b/src/modules/lists/manifest.ts
new file mode 100644
index 0000000..0c2b2ab
--- /dev/null
+++ b/src/modules/lists/manifest.ts
@@ -0,0 +1,23 @@
+import type { ModuleManifest } from "../_core/module";
+
+const manifest: ModuleManifest = {
+  id: "lists",
+  name: "Lists",
+  nav: { href: "/lists", label: "Lists", icon: "list" },
+  entities: [
+    {
+      type: "lists.list",
+      label: { singular: "List", plural: "Lists" },
+      resolveUrl: (id) => `/lists/${id}`,
+    },
+    {
+      type: "lists.item",
+      label: { singular: "List item", plural: "List items" },
+      resolveUrl: (id) => `/lists/items/${id}`,
+    },
+  ],
+  dashboardWidgets: [],
+  quickAdds: [],
+};
+
+export default manifest;
diff --git a/src/modules/notes/manifest.ts b/src/modules/notes/manifest.ts
new file mode 100644
index 0000000..9730d53
--- /dev/null
+++ b/src/modules/notes/manifest.ts
@@ -0,0 +1,18 @@
+import type { ModuleManifest } from "../_core/module";
+
+const manifest: ModuleManifest = {
+  id: "notes",
+  name: "Notes",
+  nav: { href: "/notes", label: "Notes", icon: "file-text" },
+  entities: [
+    {
+      type: "notes.note",
+      label: { singular: "Note", plural: "Notes" },
+      resolveUrl: (id) => `/notes/${id}`,
+    },
+  ],
+  dashboardWidgets: [],
+  quickAdds: [],
+};
+
+export default manifest;