refactor: remove module system integration from admin and CLI

Removes all module-related logic from the admin dashboard, CLI database
initialization, and AdminPages component:

- Drop `initModules` call from `db init` CLI command and simplify the
  completion message to only reflect core feature tables
- Remove `getModuleDashboardStats` and module page routing from admin
  stats actions and update usage documentation accordingly
- Simplify `AdminPagesClient` to remove module page loading, lazy
  components, and module-specific props (`moduleStats`, `modulePageInfo`,
  `routeInfo`, `enabledModules`)

src/features/admin/navigation.server.js

@@ -1,16 +1,6 @@
 /**
  * Admin Navigation Builder (Server-Only)
- * 
- * This file imports from the module registry and should ONLY be used on the server.
- * It builds the complete navigation including dynamic module navigation.
- * 
- * IMPORTANT: This file is NOT bundled to ensure it shares the same registry instance
- * that was populated during module discovery.
- * 
- * IMPORTANT: We import from '@zen/core' (main package) to use the same registry
- * instance that was populated during initializeZen(). DO NOT import from 
- * '@zen/core/core/modules' as that's a separate bundle with its own registry.
- * 
+ *
  * IMPORTANT: Navigation data must be serializable (no functions/components).
  * Icons are passed as string names and resolved on the client.
  */
@@ -18,7 +8,7 @@
 /**
  * Build complete navigation sections
  * @param {string} pathname - Current pathname
- * @returns {Array} Complete navigation sections (serializable, icons as strings)
+ * @returns {Array} Navigation sections (serializable, icons as strings)
  */
 export function buildNavigationSections(pathname) {
   const coreNavigation = [
@@ -36,7 +26,7 @@ export function buildNavigationSections(pathname) {
       ]
     }
   ];
-  
+
   const systemNavigation = [
     {
       id: 'users',
@@ -52,6 +42,6 @@ export function buildNavigationSections(pathname) {
       ]
     }
   ];
-  
+
   return [...coreNavigation, ...systemNavigation];
 }