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/components/layout/Sidebar.tsx

@@ -27,6 +27,7 @@ import {
   GitFork,
   Shield,
   History,
+  Server,
 } from 'lucide-react';
 import { cn } from '@/lib/utils';
 import { Button } from '@/components/ui/Button';
@@ -68,6 +69,7 @@ const navItemDefinitions: Omit<NavItem, 'label'>[] = [
   { path: '/settings', icon: Settings },
   { path: '/settings/rules', icon: Shield },
   { path: '/settings/codexlens', icon: Sparkles },
+  { path: '/api-settings', icon: Server },
   { path: '/help', icon: HelpCircle },
 ];
 
@@ -117,6 +119,7 @@ export function Sidebar({
       '/settings': 'main.settings',
       '/settings/rules': 'main.rules',
       '/settings/codexlens': 'main.codexlens',
+      '/api-settings': 'main.apiSettings',
       '/help': 'main.help',
     };
     return navItemDefinitions.map((item) => ({