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

proxy.ts

@@ -30,11 +30,17 @@ export function proxy(request: NextRequest) {
     `frame-ancestors ${frameAncestors}`,
   ].join("; ");
 
+  // Skip intl middleware for /admin routes — they have their own layout
+  const pathname = request.nextUrl.pathname;
+  const isAdminRoute = pathname === '/admin' || pathname.startsWith('/admin/');
+
   let intlResponse: ReturnType<typeof intlMiddleware> | null = null;
-  try {
-    intlResponse = intlMiddleware(request);
-  } catch (error) {
-    console.error('Locale middleware error:', error);
+  if (!isAdminRoute) {
+    try {
+      intlResponse = intlMiddleware(request);
+    } catch (error) {
+      console.error('Locale middleware error:', error);
+    }
   }
   const response = intlResponse ?? NextResponse.next();