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/claude-config.js

@@ -0,0 +1,111 @@
+/**
+ * Claude CLI Configuration Utility
+ *
+ * Handles reading and writing to the global Claude CLI settings file.
+ * Location: ~/.claude/settings.json (Windows: %USERPROFILE%\.claude\settings.json)
+ */
+
+import fs from 'fs/promises';
+import path from 'path';
+import os from 'os';
+import { logger } from './logger.js';
+
+/**
+ * Get the path to the global Claude CLI settings file
+ * @returns {string} Absolute path to settings.json
+ */
+export function getClaudeConfigPath() {
+    return path.join(os.homedir(), '.claude', 'settings.json');
+}
+
+/**
+ * Read the global Claude CLI configuration
+ * @returns {Promise<Object>} The configuration object or empty object if file missing
+ */
+export async function readClaudeConfig() {
+    const configPath = getClaudeConfigPath();
+    try {
+        const content = await fs.readFile(configPath, 'utf8');
+        if (!content.trim()) return { env: {} };
+        return JSON.parse(content);
+    } catch (error) {
+        if (error.code === 'ENOENT') {
+            logger.warn(`[ClaudeConfig] Config file not found at ${configPath}, returning empty default`);
+            return { env: {} };
+        }
+        if (error instanceof SyntaxError) {
+            logger.error(`[ClaudeConfig] Invalid JSON in config at ${configPath}. Returning safe default.`);
+            return { env: {} };
+        }
+        logger.error(`[ClaudeConfig] Failed to read config at ${configPath}:`, error.message);
+        throw error;
+    }
+}
+
+/**
+ * Update the global Claude CLI configuration
+ * Performs a deep merge with existing configuration to avoid losing other settings.
+ *
+ * @param {Object} updates - The partial configuration to merge in
+ * @returns {Promise<Object>} The updated full configuration
+ */
+export async function updateClaudeConfig(updates) {
+    const configPath = getClaudeConfigPath();
+    let currentConfig = {};
+
+    // 1. Read existing config
+    try {
+        currentConfig = await readClaudeConfig();
+    } catch (error) {
+        // Ignore ENOENT, otherwise rethrow
+        if (error.code !== 'ENOENT') throw error;
+    }
+
+    // 2. Deep merge updates
+    const newConfig = deepMerge(currentConfig, updates);
+
+    // 3. Ensure .claude directory exists
+    const configDir = path.dirname(configPath);
+    try {
+        await fs.mkdir(configDir, { recursive: true });
+    } catch (error) {
+        // Ignore if exists
+    }
+
+    // 4. Write back to file
+    try {
+        await fs.writeFile(configPath, JSON.stringify(newConfig, null, 2), 'utf8');
+        logger.info(`[ClaudeConfig] Updated config at ${configPath}`);
+        return newConfig;
+    } catch (error) {
+        logger.error(`[ClaudeConfig] Failed to write config:`, error.message);
+        throw error;
+    }
+}
+
+/**
+ * Simple deep merge for objects
+ */
+function deepMerge(target, source) {
+    const output = { ...target };
+
+    if (isObject(target) && isObject(source)) {
+        Object.keys(source).forEach(key => {
+            if (isObject(source[key])) {
+                if (!(key in target)) {
+                    Object.assign(output, { [key]: source[key] });
+                } else {
+                    output[key] = deepMerge(target[key], source[key]);
+                }
+            } else {
+                Object.assign(output, { [key]: source[key] });
+            }
+        });
+    }
+
+    return output;
+}
+
+function isObject(item) {
+    return (item && typeof item === 'object' && !Array.isArray(item));
+}