feat(storage): refactor storage config and remove module registry

Introduce a dedicated `storage-config.js` for registering public
prefixes and access policies via `configureStorageApi()`, replacing the
previous `getAllStoragePublicPrefixes` / `getAllStorageAccessPolicies`
imports from the module registry.

Remove `getAllApiRoutes()` from the router so module-level routes are no
longer auto-collected; feature routes must now be registered explicitly
via `registerFeatureRoutes()` during `initializeZen()`.

Update `.env.example` to document separate `ZEN_STORAGE_PROVIDER`,
`ZEN_STORAGE_B2_*` variables for Backblaze B2 alongside the existing
Cloudflare R2 variables, making provider selection explicit.

Clean up admin navigation and page components to drop module-injected
nav entries, keeping only core and system sections.

src/features/admin/actions.js

@@ -1,12 +1,10 @@
 /**
  * Admin Server Actions
- * 
- * These are exported separately from admin/index.js to avoid bundling
+ *
+ * Exported separately from admin/index.js to avoid bundling
  * server-side code (which includes database imports) into client components.
- * 
- * Usage:
- * import { getDashboardStats, getModuleDashboardStats } from '@zen/core/admin/actions';
+ *
+ * Usage: import { getDashboardStats } from '@zen/core/admin/actions';
  */
 
 export { getDashboardStats } from './actions/statsActions.js';
-export { getAllModuleDashboardStats as getModuleDashboardStats } from '@zen/core/modules/actions';