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

config.example.json

@@ -0,0 +1,52 @@
+{
+  "_comment": "Antigravity Claude Proxy Configuration",
+  "_instructions": [
+    "HOW TO USE THIS FILE:",
+    "1. Copy to your HOME directory: ~/.config/antigravity-proxy/config.json",
+    "   - Windows: C:\\Users\\<username>\\.config\\antigravity-proxy\\config.json",
+    "   - macOS/Linux: ~/.config/antigravity-proxy/config.json",
+    "2. Or copy to project root as 'config.json' (fallback if home config not found)",
+    "",
+    "NOTE: Environment variables (e.g., WEBUI_PASSWORD) take precedence over file config",
+    "Restart server after making changes"
+  ],
+
+  "webuiPassword": "",
+  "_webuiPassword_comment": "Optional password to protect WebUI. Can also use WEBUI_PASSWORD env var.",
+
+  "port": 8080,
+  "debug": false,
+  "logLevel": "info",
+
+  "maxRetries": 5,
+  "retryBaseMs": 1000,
+  "retryMaxMs": 30000,
+
+  "defaultCooldownMs": 60000,
+  "maxWaitBeforeErrorMs": 120000,
+
+  "tokenCacheTtlMs": 300000,
+  "persistTokenCache": false,
+
+  "requestTimeoutMs": 300000,
+  "maxAccounts": 10,
+
+  "_profiles": {
+    "development": {
+      "debug": true,
+      "logLevel": "debug",
+      "maxRetries": 3
+    },
+    "production": {
+      "debug": false,
+      "logLevel": "info",
+      "maxRetries": 5,
+      "persistTokenCache": true
+    },
+    "high-performance": {
+      "maxRetries": 10,
+      "retryMaxMs": 60000,
+      "tokenCacheTtlMs": 600000
+    }
+  }
+}