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

components/settings/settings-section.tsx

@@ -1,4 +1,5 @@
 import { ReactNode } from 'react';
+import { Lock } from 'lucide-react';
 import { cn } from '@/lib/utils';
 
 interface SettingsSectionProps {
@@ -25,18 +26,22 @@ interface SettingItemProps {
   label: string;
   description?: string;
   children: ReactNode;
+  locked?: boolean;
 }
 
-export function SettingItem({ label, description, children }: SettingItemProps) {
+export function SettingItem({ label, description, children, locked }: SettingItemProps) {
   return (
-    <div className="flex items-start justify-between py-3 border-b border-border last:border-0">
+    <div className={cn("flex items-start justify-between py-3 border-b border-border last:border-0", locked && "opacity-60")}>
       <div className="flex-1 pr-4">
-        <label className="text-sm font-medium text-foreground">{label}</label>
+        <div className="flex items-center gap-1.5">
+          <label className="text-sm font-medium text-foreground">{label}</label>
+          {locked && <Lock className="w-3 h-3 text-muted-foreground" aria-label="Managed by administrator" />}
+        </div>
         {description && (
           <p className="text-xs text-muted-foreground mt-1">{description}</p>
         )}
       </div>
-      <div className="flex-shrink-0">{children}</div>
+      <div className={cn("flex-shrink-0", locked && "pointer-events-none")}>{children}</div>
     </div>
   );
 }