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/server.ts

@@ -41,6 +41,7 @@ export interface GatewayServerConfig {
   config?: Config;
   /** Optional callback for system.restart. Should trigger graceful shutdown + process restart. */
   restart?: () => Promise<void>;
+  channelRegistry?: { list(): Array<{ readonly name: string; readonly status: string }> };
 }
 
 export class GatewayServer {
@@ -75,10 +76,18 @@ export class GatewayServer {
       getToolCount: () => this.config.toolRegistry.list().length,
       getConnectionCount: () => this.sessionBridge.connectionCount,
       restart: this.config.restart,
+      getChannels: this.config.channelRegistry
+        ? () => this.config.channelRegistry!.list().map(a => ({ name: a.name, status: a.status }))
+        : undefined,
+      getUsage: () => ({
+        totalSessions: this.config.sessionManager.listSessions().length,
+        activeConnections: this.sessionBridge.connectionCount,
+      }),
     });
 
     const sessionHandlers = createSessionHandlers({
       sessionManager: this.config.sessionManager,
+      sessionBridge: this.sessionBridge,
     });
 
     const toolHandlers = createToolHandlers({