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>

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"
   }
 }