Add module loader & registry (task 04)

Implements the extensibility foundation: ModuleManifest/DashboardWidget/EntityTypeRegistration types in _core/module.ts, a plain-Map registry singleton with registerModule/getRegistry/getEntityType/getWidget, and stub manifests for calendar/lists/notes. Root layout imports src/modules/index.ts for side-effect registration; AppNav reads the registry to render nav links. /debug/registry dumps the full registry JSON in dev using zod v4 + z.toJSONSchema(). Adding a fourth module requires one folder + one line in index.ts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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[];
+};