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/plugins/plugin-error-boundary.tsx

@@ -0,0 +1,36 @@
+'use client';
+
+import React from 'react';
+
+export interface PluginErrorBoundaryProps {
+  pluginId: string;
+  children?: React.ReactNode;
+  fallback?: React.ReactNode;
+}
+
+interface PluginErrorBoundaryState {
+  hasError: boolean;
+  error?: Error;
+}
+
+export class PluginErrorBoundary extends React.Component<PluginErrorBoundaryProps, PluginErrorBoundaryState> {
+  constructor(props: PluginErrorBoundaryProps) {
+    super(props);
+    this.state = { hasError: false };
+  }
+
+  static getDerivedStateFromError(error: Error): PluginErrorBoundaryState {
+    return { hasError: true, error };
+  }
+
+  componentDidCatch(error: Error, errorInfo: React.ErrorInfo): void {
+    console.error(`[plugin:${this.props.pluginId}] Render error:`, error, errorInfo);
+  }
+
+  render(): React.ReactNode {
+    if (this.state.hasError) {
+      return this.props.fallback ?? null;
+    }
+    return this.props.children;
+  }
+}