feat: add plugin/theme harness and admin dashboard

Plugin & Theme System:
- Add plugin type definitions, permissions (30+), and validation constants
- Add IndexedDB storage layer for plugin code, theme CSS, and previews
- Add theme CSS sanitization, injection, and safety validation
- Add HookBus event system with 130+ hooks across 20 domains
- Add plugin ZIP extraction and manifest validation with JS security checks
- Add sandboxed PluginAPI factory with scoped storage, logging, and permission gating
- Add plugin loader with blob URL dynamic import and auto-disable circuit breaker
- Add 3 built-in themes (Nord, Catppuccin, Solarized)
- Add Zustand plugin store with install/uninstall/enable/disable lifecycle
- Add PluginSlot, PluginSlotRenderer, and PluginErrorBoundary components
- Add plugins and themes settings UI panels
- Integrate plugin slots into email viewer, composer, navigation rail, sidebar, and context menu
- Extend theme store with custom theme installation and activation

Admin Dashboard:
- Add admin authentication with scrypt password hashing and AES-256-GCM sessions
- Add rate-limited login (5 attempts/15min per IP)
- Add config manager with admin override > env var > default priority
- Add settings policy system with feature gates and per-setting restrictions
- Add audit logging with rotation
- Add admin API routes (login, logout, config, policy, audit, password change)
- Add admin UI pages (login, dashboard, config, policy, audit)
- Add policy store for client-side feature gate enforcement
- Wire admin password initialization into server instrumentation

Tests:
- Add 139 tests across 10 test files covering all plugin/theme modules

app/api/settings/route.ts

@@ -4,6 +4,7 @@ import { logger } from '@/lib/logger';
 import { decryptSession } from '@/lib/auth/crypto';
 import { sessionCookieName } from '@/lib/auth/session-cookie';
 import { saveUserSettings, loadUserSettings, deleteUserSettings } from '@/lib/settings-sync';
+import { configManager } from '@/lib/admin/config-manager';
 
 function isEnabled(): boolean {
   return process.env.SETTINGS_SYNC_ENABLED === 'true' && !!process.env.SESSION_SECRET;
@@ -81,7 +82,18 @@ export async function POST(request: NextRequest) {
       return NextResponse.json({ error: 'Identity mismatch' }, { status: 403 });
     }
 
-    await saveUserSettings(username, serverUrl, settings);
+    // Enforce admin policy — strip locked settings so users can't override them
+    await configManager.ensureLoaded();
+    const policy = configManager.getPolicy();
+    const filteredSettings = { ...settings };
+    for (const key of Object.keys(filteredSettings)) {
+      const restriction = policy.restrictions[key];
+      if (restriction?.locked) {
+        delete filteredSettings[key];
+      }
+    }
+
+    await saveUserSettings(username, serverUrl, filteredSettings);
     return NextResponse.json({ ok: true });
   } catch (error) {
     const message = error instanceof Error ? error.message : 'Unknown error';