feat: add web UI dashboard SPA with dashboard, chat, sessions, and settings pages

- Add SPA shell with hash-based router, sidebar navigation, and WebSocket RPC client
- Add dashboard page with system health cards, channel status, and auto-refresh
- Add chat page with session selector, streaming tool events, and markdown rendering
- Add sessions page with list, history viewer, and delete functionality
- Add settings page with hook pattern editor, tool list, and config viewer
- Add backend handlers: sessions.delete, sessions.switch, system.channels, system.usage
- Wire channelRegistry into gateway server for channel status reporting
- Extend static file server with .mjs, .png, .ico, .woff2 content types

src/gateway/static.ts

@@ -7,8 +7,12 @@ const CONTENT_TYPES: Record<string, string> = {
   '.html': 'text/html',
   '.css': 'text/css',
   '.js': 'application/javascript',
+  '.mjs': 'application/javascript',
   '.json': 'application/json',
   '.svg': 'image/svg+xml',
+  '.png': 'image/png',
+  '.ico': 'image/x-icon',
+  '.woff2': 'font/woff2',
 };
 
 /**