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/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;