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/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);