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/zh.json

@@ -441,6 +441,22 @@
     "claudeTitle": "Claude Code MCP 管理",
     "codexTitle": "Codex MCP 管理",
     "geminiTitle": "Gemini MCP 管理",
+    "unifiedPanel": {
+      "title": "MCP 服务器管理",
+      "addServer": "添加服务器",
+      "editServer": "编辑服务器",
+      "deleteServer": "删除服务器",
+      "deleteConfirm": "确定要删除服务器 \"{{id}}\" 吗？此操作无法撤销。",
+      "syncAll": "同步全部",
+      "syncAllSuccess": "已同步所有启用的服务器到各应用配置",
+      "noServers": "暂无服务器",
+      "enabledApps": "启用的应用",
+      "apps": {
+        "claude": "Claude",
+        "codex": "Codex",
+        "gemini": "Gemini"
+      }
+    },
     "userLevelPath": "用户级 MCP 配置路径",
     "serverList": "服务器列表",
     "loading": "加载中...",