feat: Add Web UI for account and quota management

## Summary
Add an optional Web UI for managing accounts and monitoring quotas.
WebUI is implemented as a modular plugin with minimal changes to server.js (only 5 lines added).

## New Features
- Dashboard: Real-time model quota visualization with Chart.js
- Accounts: OAuth-based account management (add/enable/disable/refresh/remove)
- Logs: Live server log streaming via SSE with search and level filtering
- Settings: System configuration with 4 tabs
  - Interface: Language (EN/zh_CN), polling interval, log buffer size, display options
  - Claude CLI: Proxy connection config, model selection, alias overrides (~/.claude.json)
  - Models: Model visibility and ordering management
  - Server Info: Runtime info and account config reload

## Technical Changes
- Add src/webui/index.js as modular plugin (all WebUI routes encapsulated)
- Add src/config.js for centralized configuration (~/.config/antigravity-proxy/config.json)
- Add authMiddleware for optional password protection (WEBUI_PASSWORD env var)
- Enhance logger with EventEmitter for SSE log streaming
- Make constants configurable via config.json
- Merge with main v1.2.6 (model fallback, cross-model thinking)
- server.js changes: only 5 lines added to import and mount WebUI module

## Bug Fixes
- Fix Alpine.js $watch error in settings-store.js (not supported in store init)
- Fix "OK" label to "SUCCESS" in logs filter
- Add saveSettings() calls to settings toggles for proper persistence
- Improve Claude CLI config robustness (handle empty/invalid JSON files)
- Add safety check for empty config.env in claude-config component
- Improve config.example.json instructions with clear Windows/macOS/Linux paths

## New Files
- src/webui/index.js - WebUI module with all API routes
- public/ - Complete Web UI frontend (Alpine.js + TailwindCSS + DaisyUI)
- src/config.js - Configuration management
- src/utils/claude-config.js - Claude CLI settings helper
- tests/frontend/ - Frontend test suite

## API Endpoints Added
- GET/POST /api/config - Server configuration
- GET/POST /api/claude/config - Claude CLI configuration
- POST /api/models/config - Model alias/hidden settings
- GET /api/accounts - Account list with status
- POST /api/accounts/:email/toggle - Enable/disable account
- POST /api/accounts/:email/refresh - Refresh account token
- DELETE /api/accounts/:email - Remove account
- GET /api/logs - Log history
- GET /api/logs/stream - Live log streaming (SSE)
- GET /api/auth/url - OAuth URL generation
- GET /oauth/callback - OAuth callback handler

## Backward Compatibility
- Default port remains 8080
- All existing CLI/API functionality unchanged
- WebUI is entirely optional
- Can be disabled by removing mountWebUI() call

public/css/style.css

@@ -0,0 +1,92 @@
+[x-cloak] {
+    display: none !important;
+}
+
+/* Custom Scrollbar */
+::-webkit-scrollbar {
+    width: 6px;
+    height: 6px;
+}
+
+::-webkit-scrollbar-track {
+    background: transparent;
+}
+
+::-webkit-scrollbar-thumb {
+    background: #3f3f46;
+    border-radius: 3px;
+}
+
+::-webkit-scrollbar-thumb:hover {
+    background: #52525b;
+}
+
+/* Animations */
+.fade-enter-active,
+.fade-leave-active {
+    transition: opacity 0.2s ease;
+}
+
+.fade-enter-from,
+.fade-leave-to {
+    opacity: 0;
+}
+
+@keyframes fadeIn {
+    from { opacity: 0; transform: translateY(5px); }
+    to { opacity: 1; transform: translateY(0); }
+}
+
+.animate-fade-in {
+    animation: fadeIn 0.4s ease-out forwards;
+}
+
+/* Utility */
+.glass-panel {
+    background: rgba(23, 23, 23, 0.7);
+    backdrop-filter: blur(12px);
+    border: 1px solid rgba(255, 255, 255, 0.08);
+}
+
+.nav-item.active {
+    background: linear-gradient(90deg, rgba(168, 85, 247, 0.15) 0%, transparent 100%);
+    border-left: 3px solid #a855f7;
+    color: #fff;
+}
+
+.nav-item {
+    border-left: 3px solid transparent;
+    transition: all 0.2s;
+}
+
+.progress-gradient-success::-webkit-progress-value {
+    background-image: linear-gradient(to right, #22c55e, #4ade80);
+}
+
+.progress-gradient-warning::-webkit-progress-value {
+    background-image: linear-gradient(to right, #eab308, #facc15);
+}
+
+.progress-gradient-error::-webkit-progress-value {
+    background-image: linear-gradient(to right, #dc2626, #f87171);
+}
+
+/* Dashboard Grid */
+.stats-grid {
+    display: grid;
+    grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
+    gap: 1.5rem;
+}
+
+/* Tooltip Customization */
+.tooltip:before {
+    background-color: #171717 !important; /* space-800 */
+    border: 1px solid #27272a; /* space-border */
+    color: #e5e7eb !important;
+    font-family: 'JetBrains Mono', monospace;
+    font-size: 0.75rem;
+}
+
+.tooltip-left:before {
+    margin-right: 0.5rem;
+}