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/i18n/locales/en.json

@@ -441,6 +441,22 @@
     "claudeTitle": "Claude Code MCP Management",
     "codexTitle": "Codex MCP Management",
     "geminiTitle": "Gemini MCP Management",
+    "unifiedPanel": {
+      "title": "MCP Server Management",
+      "addServer": "Add Server",
+      "editServer": "Edit Server",
+      "deleteServer": "Delete Server",
+      "deleteConfirm": "Are you sure you want to delete server \"{{id}}\"? This action cannot be undone.",
+      "syncAll": "Sync All",
+      "syncAllSuccess": "All enabled servers have been synced to application configs",
+      "noServers": "No servers yet",
+      "enabledApps": "Enabled Apps",
+      "apps": {
+        "claude": "Claude",
+        "codex": "Codex",
+        "gemini": "Gemini"
+      }
+    },
     "userLevelPath": "User-level MCP path",
     "serverList": "Servers",
     "loading": "Loading...",