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/js/components/claude-config.js

@@ -0,0 +1,54 @@
+/**
+ * Claude Config Component
+ * Registers itself to window.Components for Alpine.js to consume
+ */
+window.Components = window.Components || {};
+
+window.Components.claudeConfig = () => ({
+    config: { env: {} },
+    models: [],
+    loading: false,
+
+    init() {
+        this.fetchConfig();
+        this.$watch('$store.data.models', (val) => {
+            this.models = val || [];
+        });
+        this.models = Alpine.store('data').models || [];
+    },
+
+    async fetchConfig() {
+        const password = Alpine.store('global').webuiPassword;
+        try {
+            const { response, newPassword } = await window.utils.request('/api/claude/config', {}, password);
+            if (newPassword) Alpine.store('global').webuiPassword = newPassword;
+
+            if (!response.ok) throw new Error(`HTTP ${response.status}`);
+            const data = await response.json();
+            this.config = data.config || {};
+            if (!this.config.env) this.config.env = {};
+        } catch (e) {
+            console.error('Failed to fetch Claude config:', e);
+        }
+    },
+
+    async saveClaudeConfig() {
+        this.loading = true;
+        const password = Alpine.store('global').webuiPassword;
+        try {
+            const { response, newPassword } = await window.utils.request('/api/claude/config', {
+                method: 'POST',
+                headers: { 'Content-Type': 'application/json' },
+                body: JSON.stringify(this.config)
+            }, password);
+            if (newPassword) Alpine.store('global').webuiPassword = newPassword;
+
+            if (!response.ok) throw new Error(`HTTP ${response.status}`);
+            Alpine.store('global').showToast('Claude config saved!', 'success');
+        } catch (e) {
+            Alpine.store('global').showToast('Failed to save config: ' + e.message, 'error');
+        } finally {
+            this.loading = false;
+        }
+    }
+});