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>

frontend/playwright.config.ts

@@ -0,0 +1,50 @@
+import { defineConfig, devices } from "@playwright/test";
+import { resolve } from "node:path";
+
+const __dir = new URL(".", import.meta.url).pathname;
+const fakePi = resolve(__dir, "e2e/fixtures/fake-pi.mjs");
+const fakeTht = resolve(__dir, "e2e/fixtures/fake-tht.mjs");
+const harnessDir = resolve(__dir, "..", "harness");
+
+export default defineConfig({
+  testDir: "./e2e",
+  timeout: 60_000,
+  retries: process.env.CI ? 2 : 0,
+  use: {
+    baseURL: "http://localhost:5199",
+  },
+  projects: [
+    {
+      name: "chromium",
+      use: { ...devices["Desktop Chrome"] },
+    },
+  ],
+  webServer: [
+    {
+      // Backend — wired to both fakes; no VPN/Pi/Python required.
+      command: "npm run dev",
+      cwd: resolve(__dir, "..", "backend"),
+      url: "http://localhost:8799/health",
+      timeout: 30_000,
+      reuseExistingServer: !process.env.CI,
+      env: {
+        PI_BIN: fakePi,
+        THT_BIN: fakeTht,
+        THT_HARNESS_DIR: harnessDir,
+        PORT: "8799",
+        AUTH_MODE: "none",
+      },
+    },
+    {
+      // Frontend dev server pointing at the hermetic backend.
+      command: "npm run dev -- --port 5199",
+      cwd: __dir,
+      url: "http://localhost:5199",
+      timeout: 30_000,
+      reuseExistingServer: !process.env.CI,
+      env: {
+        VITE_BACKEND_URL: "http://localhost:8799",
+      },
+    },
+  ],
+});