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/package-lock.json

@@ -27,6 +27,7 @@
         "zustand": "^5.0.0"
       },
       "devDependencies": {
+        "@playwright/test": "^1.61.1",
         "@testing-library/jest-dom": "^6.5.0",
         "@testing-library/react": "^16.0.0",
         "@testing-library/user-event": "^14.5.0",
@@ -1788,6 +1789,22 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@playwright/test": {
+      "version": "1.61.1",
+      "resolved": "https://registry.npmjs.org/@playwright/test/-/test-1.61.1.tgz",
+      "integrity": "sha512-8nKv6+0RJSL9FE4jYOEGXnPeM/Hg12qZpmqzZjRh3qM0Y7c3z1mrOTfFLids72RDQYVh9WpLEfR5WdpNX4fkig==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "dependencies": {
+        "playwright": "1.61.1"
+      },
+      "bin": {
+        "playwright": "cli.js"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    },
     "node_modules/@rolldown/pluginutils": {
       "version": "1.0.0-beta.27",
       "resolved": "https://registry.npmjs.org/@rolldown/pluginutils/-/pluginutils-1.0.0-beta.27.tgz",
@@ -8298,6 +8315,53 @@
         "node": ">=8"
       }
     },
+    "node_modules/playwright": {
+      "version": "1.61.1",
+      "resolved": "https://registry.npmjs.org/playwright/-/playwright-1.61.1.tgz",
+      "integrity": "sha512-DWnY5o3YbLWK4GovuAVwpqL+1VwGNdUGrRr++8j8PtQQzvAVZUIMjKQ90fY689sEJZJBbZVw1rXaOKSTitkzPQ==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "dependencies": {
+        "playwright-core": "1.61.1"
+      },
+      "bin": {
+        "playwright": "cli.js"
+      },
+      "engines": {
+        "node": ">=18"
+      },
+      "optionalDependencies": {
+        "fsevents": "2.3.2"
+      }
+    },
+    "node_modules/playwright-core": {
+      "version": "1.61.1",
+      "resolved": "https://registry.npmjs.org/playwright-core/-/playwright-core-1.61.1.tgz",
+      "integrity": "sha512-h7Qlt6m4REp25qvIdvbDtVmD4LqVXfpRxhORv9L0jzETM05p4fuPJ3dKyuSXQxDSbXnmS79HAgi9589lGSpLkg==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "bin": {
+        "playwright-core": "cli.js"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    },
+    "node_modules/playwright/node_modules/fsevents": {
+      "version": "2.3.2",
+      "resolved": "https://registry.npmjs.org/fsevents/-/fsevents-2.3.2.tgz",
+      "integrity": "sha512-xiqMQR4xAeHTuB9uWm+fFRcIOgKBMiOBP+eXiyT7jsgVCq1bkVygt00oASowB7EdtpOHaaPgKt812P9ab+DDKA==",
+      "dev": true,
+      "hasInstallScript": true,
+      "license": "MIT",
+      "optional": true,
+      "os": [
+        "darwin"
+      ],
+      "engines": {
+        "node": "^8.16.0 || ^10.6.0 || >=11.0.0"
+      }
+    },
     "node_modules/points-on-curve": {
       "version": "0.2.0",
       "resolved": "https://registry.npmjs.org/points-on-curve/-/points-on-curve-0.2.0.tgz",