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/utils/logger.js

@@ -1,15 +1,18 @@
 /**
  * Logger Utility
- * 
+ *
  * Provides structured logging with colors and debug support.
  * Simple ANSI codes used to avoid dependencies.
  */
 
+import { EventEmitter } from 'events';
+import util from 'util';
+
 const COLORS = {
     RESET: '\x1b[0m',
     BRIGHT: '\x1b[1m',
     DIM: '\x1b[2m',
-    
+
     RED: '\x1b[31m',
     GREEN: '\x1b[32m',
     YELLOW: '\x1b[33m',
@@ -20,14 +23,17 @@ const COLORS = {
     GRAY: '\x1b[90m'
 };
 
-class Logger {
+class Logger extends EventEmitter {
     constructor() {
+        super();
         this.isDebugEnabled = false;
+        this.history = [];
+        this.maxHistory = 1000;
     }
 
     /**
      * Set debug mode
-     * @param {boolean} enabled 
+     * @param {boolean} enabled
      */
     setDebug(enabled) {
         this.isDebugEnabled = !!enabled;
@@ -40,19 +46,44 @@ class Logger {
         return new Date().toISOString();
     }
 
+    /**
+     * Get log history
+     */
+    getHistory() {
+        return this.history;
+    }
+
     /**
      * Format and print a log message
-     * @param {string} level 
-     * @param {string} color 
-     * @param {string} message 
-     * @param  {...any} args 
+     * @param {string} level
+     * @param {string} color
+     * @param {string} message
+     * @param  {...any} args
      */
     print(level, color, message, ...args) {
         // Format: [TIMESTAMP] [LEVEL] Message
-        const timestamp = `${COLORS.GRAY}[${this.getTimestamp()}]${COLORS.RESET}`;
+        const timestampStr = this.getTimestamp();
+        const timestamp = `${COLORS.GRAY}[${timestampStr}]${COLORS.RESET}`;
         const levelTag = `${color}[${level}]${COLORS.RESET}`;
-        
-        console.log(`${timestamp} ${levelTag} ${message}`, ...args);
+
+        // Format the message with args similar to console.log
+        const formattedMessage = util.format(message, ...args);
+
+        console.log(`${timestamp} ${levelTag} ${formattedMessage}`);
+
+        // Store structured log
+        const logEntry = {
+            timestamp: timestampStr,
+            level,
+            message: formattedMessage
+        };
+
+        this.history.push(logEntry);
+        if (this.history.length > this.maxHistory) {
+            this.history.shift();
+        }
+
+        this.emit('log', logEntry);
     }
 
     /**
@@ -98,7 +129,7 @@ class Logger {
     log(message, ...args) {
         console.log(message, ...args);
     }
-    
+
     /**
      * Print a section header
      */