fix(web): conversation DELETE — resolve Failed to fetch TypeError

Root cause: @fastify/cors default allowed methods are only GET, HEAD, POST
(CORS-safelisted methods). DELETE requires a CORS preflight OPTIONS request,
and without DELETE in Access-Control-Allow-Methods the browser rejects it
with TypeError: Failed to fetch before the request reaches the server.

Fix: explicitly set methods in enableCors() to include DELETE and all other
HTTP verbs used by the API.

Also add try/catch to handleDelete in ChatPage so errors surface in the
console rather than becoming unhandled promise rejections.

Fixes #195.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/gateway/src/main.ts

@@ -40,6 +40,7 @@ async function bootstrap(): Promise<void> {
   app.enableCors({
     origin: process.env['GATEWAY_CORS_ORIGIN'] ?? 'http://localhost:3000',
     credentials: true,
+    methods: ['GET', 'HEAD', 'POST', 'PUT', 'PATCH', 'DELETE', 'OPTIONS'],
   });
 
   await app.register(helmet as never, { contentSecurityPolicy: false });