refactor(frontend): remove redundant 'Sync All' button from MCP panel

All MCP operations already auto-sync to live configs:
- upsert_server() → sync_server_to_apps()
- toggle_app() → sync_server_to_app() or remove_server_from_app()
- delete_server() → remove_server_from_all_apps()

The manual 'Sync All' button was redundant and could confuse users
into thinking they need to manually sync after each change.

Changes:
- Remove 'Sync All' button from UnifiedMcpPanel header
- Remove useSyncAllMcpServers hook
- Remove handleSyncAll function and syncAllMutation state
- Remove RefreshCw icon import
- Remove sync-related i18n translations (en/zh)

Note: Backend sync_all_mcp_servers command remains for potential
future use (e.g., recovery tool), but is no longer exposed in UI.

src/i18n/locales/zh.json

@@ -447,8 +447,6 @@
       "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": {