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)

tests/integration/McpPanel.test.tsx

@@ -36,6 +36,7 @@ const baseServers: Record<string, McpServer> = {
     id: "sample",
     name: "Sample Claude Server",
     enabled: true,
+    apps: { claude: true, codex: false, gemini: false },
     server: {
       type: "stdio",
       command: "claude-server",