test(e2e): Playwright UI smoke + Go integration tests against MAS

- Playwright e2e against Vite dev server (LoginScreen renders, homeserver
  visible, version pinned). Fallback documented to swap to `wails dev
  -browser` via WAILS_DEV_URL when full backend bindings are required.
- Go integration tests (//go:build integration,goolm) for Start +
  ListRooms + LoadTimeline + SendText + GetDiagnostics against real
  Synapse. Skip cleanly if MATRIX_INTEGRATION_TOKEN/USER env vars unset
  so CI never breaks unattended.
- pnpm e2e/e2e:ui/e2e:report scripts. Per-run keyring namespace +
  t.TempDir XDG_CONFIG_HOME so integration tests don't clobber the
  production keyring entry.

E2E.md explains how to run each layer + limits (OIDC not automated).

frontend/package.json

@@ -9,7 +9,10 @@
     "preview": "vite preview",
     "test": "vitest run",
     "test:watch": "vitest",
-    "test:ui": "vitest --ui"
+    "test:ui": "vitest --ui",
+    "e2e": "playwright test",
+    "e2e:ui": "playwright test --ui",
+    "e2e:report": "playwright show-report"
   },
   "dependencies": {
     "@mantine/core": "^7.13.0",
@@ -20,6 +23,7 @@
     "react-dom": "^18.3.1"
   },
   "devDependencies": {
+    "@playwright/test": "^1.60.0",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.2",
     "@testing-library/user-event": "^14.6.1",