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

src/config.js

@@ -0,0 +1,84 @@
+import fs from 'fs';
+import path from 'path';
+import os from 'os';
+import { logger } from './utils/logger.js';
+
+// Default config
+const DEFAULT_CONFIG = {
+    webuiPassword: '',
+    debug: false,
+    logLevel: 'info',
+    maxRetries: 5,
+    retryBaseMs: 1000,
+    retryMaxMs: 30000,
+    persistTokenCache: false,
+    modelMapping: {}
+};
+
+// Config locations
+const HOME_DIR = os.homedir();
+const CONFIG_DIR = path.join(HOME_DIR, '.config', 'antigravity-proxy');
+const CONFIG_FILE = path.join(CONFIG_DIR, 'config.json');
+
+// Ensure config dir exists
+if (!fs.existsSync(CONFIG_DIR)) {
+    try {
+        fs.mkdirSync(CONFIG_DIR, { recursive: true });
+    } catch (err) {
+        // Ignore
+    }
+}
+
+// Load config
+let config = { ...DEFAULT_CONFIG };
+
+function loadConfig() {
+    try {
+        // Env vars take precedence for initial defaults, but file overrides them if present?
+        // Usually Env > File > Default.
+
+        if (fs.existsSync(CONFIG_FILE)) {
+            const fileContent = fs.readFileSync(CONFIG_FILE, 'utf8');
+            const userConfig = JSON.parse(fileContent);
+            config = { ...DEFAULT_CONFIG, ...userConfig };
+        } else {
+             // Try looking in current dir for config.json as fallback
+             const localConfigPath = path.resolve('config.json');
+             if (fs.existsSync(localConfigPath)) {
+                 const fileContent = fs.readFileSync(localConfigPath, 'utf8');
+                 const userConfig = JSON.parse(fileContent);
+                 config = { ...DEFAULT_CONFIG, ...userConfig };
+             }
+        }
+
+        // Environment overrides
+        if (process.env.WEBUI_PASSWORD) config.webuiPassword = process.env.WEBUI_PASSWORD;
+        if (process.env.DEBUG === 'true') config.debug = true;
+
+    } catch (error) {
+        console.error('[Config] Error loading config:', error);
+    }
+}
+
+// Initial load
+loadConfig();
+
+export function getPublicConfig() {
+    return { ...config };
+}
+
+export function saveConfig(updates) {
+    try {
+        // Apply updates
+        config = { ...config, ...updates };
+
+        // Save to disk
+        fs.writeFileSync(CONFIG_FILE, JSON.stringify(config, null, 2), 'utf8');
+        return true;
+    } catch (error) {
+        logger.error('[Config] Failed to save config:', error);
+        return false;
+    }
+}
+
+export { config };