test(frontend): Playwright e2e F1 vs backend+fake-pi (hermetic)

Adds a real-browser Playwright e2e of the full F1 disambiguation loop,
driven against the real backend + fake binaries (no VPN, no real Pi, no Python).

- frontend/e2e/fixtures/fake-tht.mjs: stubs tht CLI (session new/list/show)
- frontend/e2e/fixtures/fake-pi.mjs: wraps harness fake-pi-rpc with f1_disambiguation.json
- frontend/playwright.config.ts: two webServer entries (backend:8799, frontend:5199)
- frontend/e2e/f1.spec.ts: open app → create session → wait for SelectWidget → respond

Bug fixes discovered during e2e:
- useSessionStream: add addEventListener for named SSE events (backend sends
  'event: ui_request' etc.; onmessage only fires for unnamed 'event: message')
- FakeEventSource: add no-op addEventListener/removeEventListener stubs
- backend SSE route: add CORS headers manually in writeHead() since
  reply.raw bypasses the @fastify/cors onSend hook
- backend: install and register @fastify/cors for all non-SSE routes

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

backend/src/routes/sessions.ts

@@ -44,7 +44,16 @@ export function sessionRoutes(app: FastifyInstance, d: { mgr: PiProcessManager;
   app.get("/sessions/:id/events", (req, reply) => {
     const id = (req.params as any).id;
     const rt = d.mgr.get(id);
-    reply.raw.writeHead(200, { "Content-Type": "text/event-stream", "Cache-Control": "no-cache", Connection: "keep-alive" });
+    // Add CORS headers manually: reply.raw.writeHead bypasses Fastify's onSend hooks
+    // (where @fastify/cors injects headers), so we must set them explicitly here.
+    const origin = (req.headers.origin as string | undefined) ?? "*";
+    reply.raw.writeHead(200, {
+      "Content-Type": "text/event-stream",
+      "Cache-Control": "no-cache",
+      Connection: "keep-alive",
+      "Access-Control-Allow-Origin": origin,
+      "Access-Control-Allow-Credentials": "true",
+    });
     const send = (event: string, data: object) => reply.raw.write(`event: ${event}\ndata: ${JSON.stringify(data)}\n\n`);
     const off = d.hub.subscribe(id, send, rt?.bridge.pendingWidget() ?? null);
     req.raw.on("close", off);