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

stores/policy-store.ts

@@ -0,0 +1,55 @@
+import { create } from 'zustand';
+import type { SettingsPolicy, FeatureGates, SettingRestriction } from '@/lib/admin/types';
+import { DEFAULT_POLICY } from '@/lib/admin/types';
+
+interface PolicyState {
+  policy: SettingsPolicy;
+  loaded: boolean;
+  fetchPolicy: () => Promise<void>;
+  isSettingLocked: (key: string) => boolean;
+  isSettingHidden: (key: string) => boolean;
+  isFeatureEnabled: (feature: keyof FeatureGates) => boolean;
+  getRestriction: (key: string) => SettingRestriction | undefined;
+  getEffectiveDefault: (key: string) => unknown;
+}
+
+export const usePolicyStore = create<PolicyState>()((set, get) => ({
+  policy: { ...DEFAULT_POLICY },
+  loaded: false,
+
+  fetchPolicy: async () => {
+    try {
+      const res = await fetch('/api/admin/policy');
+      if (res.ok) {
+        const data = await res.json();
+        set({ policy: data, loaded: true });
+      } else {
+        set({ loaded: true });
+      }
+    } catch {
+      set({ loaded: true });
+    }
+  },
+
+  isSettingLocked: (key) => {
+    const r = get().policy.restrictions[key];
+    return r?.locked === true;
+  },
+
+  isSettingHidden: (key) => {
+    const r = get().policy.restrictions[key];
+    return r?.hidden === true;
+  },
+
+  isFeatureEnabled: (feature) => {
+    return get().policy.features[feature] ?? true;
+  },
+
+  getRestriction: (key) => {
+    return get().policy.restrictions[key];
+  },
+
+  getEffectiveDefault: (key) => {
+    return get().policy.defaults[key];
+  },
+}));