feat(frontend): implement comprehensive API Settings Management Interface

Implement a complete API Management Interface for React frontend with split-
panel layout, migrating all features from legacy JS frontend.

New Features:
- API Settings page with 5 tabs: Providers, Endpoints, Cache, Model Pools, CLI Settings
- Provider Management: CRUD operations, multi-key rotation, health checks, test connection
- Endpoint Management: CRUD operations, cache strategy configuration, enable/disable toggle
- Cache Settings: Global configuration, statistics display, clear cache functionality
- Model Pool Management: CRUD operations, auto-discovery feature, provider exclusion
- CLI Settings Management: Provider-based and Direct modes, full CRUD support
- Multi-Key Settings Modal: Manage API keys with rotation strategies and weights
- Manage Models Modal: View and manage models per provider (LLM and Embedding)
- Sync to CodexLens: Integration handler for provider configuration sync

Technical Implementation:
- Created 12 new React components in components/api-settings/
- Extended lib/api.ts with 460+ lines of API client functions
- Created hooks/useApiSettings.ts with 772 lines of TanStack Query hooks
- Added RadioGroup UI component for form selections
- Implemented unified error handling with useNotifications across all operations
- Complete i18n support (500+ keys in English and Chinese)
- Route integration (/api-settings) and sidebar navigation

Code Quality:
- All acceptance criteria from plan.json verified
- Code review passed with Gemini (all 7 IMPL tasks complete)
- Follows existing patterns: Shadcn UI, TanStack Query, react-intl, Lucide icons

ccw/frontend/src/router.tsx

@@ -37,6 +37,7 @@ import {
   ExplorerPage,
   GraphExplorerPage,
   CodexLensManagerPage,
+  ApiSettingsPage,
 } from '@/pages';
 
 /**
@@ -146,6 +147,10 @@ const routes: RouteObject[] = [
         path: 'settings/codexlens',
         element: <CodexLensManagerPage />,
       },
+      {
+        path: 'api-settings',
+        element: <ApiSettingsPage />,
+      },
       {
         path: 'help',
         element: <HelpPage />,
@@ -212,6 +217,7 @@ export const ROUTES = {
   INSTALLATIONS: '/settings/installations',
   SETTINGS_RULES: '/settings/rules',
   CODEXLENS_MANAGER: '/settings/codexlens',
+  API_SETTINGS: '/api-settings',
   HELP: '/help',
   EXPLORER: '/explorer',
   GRAPH: '/graph',