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/constants.js

@@ -5,6 +5,7 @@
 
 import { homedir, platform, arch } from 'os';
 import { join } from 'path';
+import { config } from './config.js';
 
 /**
  * Get the Antigravity database path based on the current platform.
@@ -59,13 +60,14 @@ export const ANTIGRAVITY_HEADERS = {
 // Default project ID if none can be discovered
 export const DEFAULT_PROJECT_ID = 'rising-fact-p41fc';
 
-export const TOKEN_REFRESH_INTERVAL_MS = 5 * 60 * 1000; // 5 minutes
-export const REQUEST_BODY_LIMIT = '50mb';
+// Configurable constants - values from config.json take precedence
+export const TOKEN_REFRESH_INTERVAL_MS = config?.tokenCacheTtlMs || (5 * 60 * 1000); // From config or 5 minutes
+export const REQUEST_BODY_LIMIT = config?.requestBodyLimit || '50mb';
 export const ANTIGRAVITY_AUTH_PORT = 9092;
-export const DEFAULT_PORT = 8080;
+export const DEFAULT_PORT = config?.port || 8080;
 
 // Multi-account configuration
-export const ACCOUNT_CONFIG_PATH = join(
+export const ACCOUNT_CONFIG_PATH = config?.accountConfigPath || join(
     homedir(),
     '.config/antigravity-proxy/accounts.json'
 );
@@ -74,12 +76,12 @@ export const ACCOUNT_CONFIG_PATH = join(
 // Uses platform-specific path detection
 export const ANTIGRAVITY_DB_PATH = getAntigravityDbPath();
 
-export const DEFAULT_COOLDOWN_MS = 60 * 1000; // 1 minute default cooldown
-export const MAX_RETRIES = 5; // Max retry attempts across accounts
-export const MAX_ACCOUNTS = 10; // Maximum number of accounts allowed
+export const DEFAULT_COOLDOWN_MS = config?.defaultCooldownMs || (60 * 1000); // From config or 1 minute
+export const MAX_RETRIES = config?.maxRetries || 5; // From config or 5
+export const MAX_ACCOUNTS = config?.maxAccounts || 10; // From config or 10
 
 // Rate limit wait thresholds
-export const MAX_WAIT_BEFORE_ERROR_MS = 120000; // 2 minutes - throw error if wait exceeds this
+export const MAX_WAIT_BEFORE_ERROR_MS = config?.maxWaitBeforeErrorMs || 120000; // From config or 2 minutes
 
 // Thinking model constants
 export const MIN_SIGNATURE_LENGTH = 50; // Minimum valid thinking signature length