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>

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