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/layout/sidebar.tsx

@@ -3,6 +3,7 @@
 import { useState, useEffect } from "react";
 import { useTranslations } from "next-intl";
 import { useRouter } from "@/i18n/navigation";
+import { PluginSlot } from "@/components/plugins/plugin-slot";
 import { Button } from "@/components/ui/button";
 import {
   Inbox,
@@ -41,8 +42,6 @@ import { useSettingsStore, KEYWORD_PALETTE, KeywordDefinition } from "@/stores/s
 import { useEmailStore } from "@/stores/email-store";
 import { toast } from "@/stores/toast-store";
 import { debug } from "@/lib/debug";
-import { useConfig } from "@/hooks/use-config";
-import { useThemeStore } from "@/stores/theme-store";
 import { AccountSwitcher } from "./account-switcher";
 import { useTour } from "@/components/tour/tour-provider";
 
@@ -431,8 +430,6 @@ export function Sidebar({
 }: SidebarProps) {
   const { sidebarCollapsed: isCollapsed, toggleSidebarCollapsed } = useUIStore();
   const { primaryIdentity } = useAuthStore();
-  const { appLogoLightUrl, appLogoDarkUrl } = useConfig();
-  const resolvedTheme = useThemeStore((s) => s.resolvedTheme);
   const [expandedFolders, setExpandedFolders] = useState<Set<string>>(new Set());
   const [tagsExpanded, setTagsExpanded] = useState(() => {
     try {
@@ -532,17 +529,6 @@ export function Sidebar({
           <X className="w-5 h-5" />
         </Button>
 
-        {(() => {
-          const logoUrl = resolvedTheme === 'dark' ? (appLogoDarkUrl || appLogoLightUrl) : (appLogoLightUrl || appLogoDarkUrl);
-          return logoUrl ? (
-            <img
-              src={logoUrl}
-              alt=""
-              className={cn("object-contain flex-shrink-0", isCollapsed ? "w-6 h-6" : "w-6 h-6")}
-            />
-          ) : null;
-        })()}
-
         <Button
           variant="ghost"
           size="icon"
@@ -673,6 +659,8 @@ export function Sidebar({
         )}
       </div>
 
+      <PluginSlot name="sidebar-widget" />
+
       {/* Compose Button */}
       <div className={cn("border-t border-border", isCollapsed ? "flex justify-center py-3" : "px-3 py-3")}>
         {isCollapsed ? (