feat(frontend): implement unified MCP panel for v3.7.0

Complete Phase 3 (P0) frontend implementation for unified MCP management:

**New Files:**
- src/hooks/useMcp.ts: React Query hooks for unified MCP operations
- src/components/mcp/UnifiedMcpPanel.tsx: Unified MCP management panel
- src/components/ui/checkbox.tsx: Checkbox component from shadcn/ui

**Features:**
- Unified panel with three-column layout: server info + app checkboxes + actions
- Multi-app control: Claude/Codex/Gemini checkboxes for each server
- Real-time stats: Show enabled server counts per app
- Full CRUD operations: Add, edit, delete, sync all servers

**Integration:**
- Replace old app-specific McpPanel with UnifiedMcpPanel in App.tsx
- Update McpFormModal to support unified mode with apps field
- Add i18n support: mcp.unifiedPanel namespace (zh/en)

**Type Safety:**
- Ensure McpServer.apps field always initialized
- Fix all test files to include apps field
- TypeScript type check passes ✅

**Architecture:**
- Single source of truth: mcp.servers manages all MCP configs
- Per-server app control: apps.claude/codex/gemini boolean flags
- Backward compatible: McpFormModal supports both unified and legacy modes

Next: P1 tasks (import dialogs, sub-components, tests)

src/App.tsx

@@ -20,7 +20,7 @@ import { ConfirmDialog } from "@/components/ConfirmDialog";
 import { SettingsDialog } from "@/components/settings/SettingsDialog";
 import { UpdateBadge } from "@/components/UpdateBadge";
 import UsageScriptModal from "@/components/UsageScriptModal";
-import McpPanel from "@/components/mcp/McpPanel";
+import UnifiedMcpPanel from "@/components/mcp/UnifiedMcpPanel";
 import PromptPanel from "@/components/prompts/PromptPanel";
 import { Button } from "@/components/ui/button";
 
@@ -302,10 +302,9 @@ function App() {
         appId={activeApp}
       />
 
-      <McpPanel
+      <UnifiedMcpPanel
         open={isMcpOpen}
         onOpenChange={setIsMcpOpen}
-        appId={activeApp}
       />
     </div>
   );