feat(desktop/e2e): Playwright E2E suite with visual regression diffs

Adds a full desktop Playwright E2E suite that launches the Electron app
against a mock inference server, exercising the full boot chain:

  electron -> hermes serve -> mock provider -> renderer

Includes:
- Mock OpenAI-compatible inference server (mock-server.ts)
- Shared fixtures with sandbox isolation (credentials, HERMES_HOME,
  userData, fixed window-state.json for reproducible screenshots)
- Test specs: boot, boot-failure, onboarding, mock-backend-setup, chat,
  and packaged-app launch
- Visual regression: expectVisualSnapshot() wraps toHaveScreenshot in
  try/catch so diffs are reported without failing the test suite
- CI workflow: xvfb at 1280x1024, baseline cache from main
  (--update-snapshots on main, compare on PRs), step summary table with
  diff/actual/expected image links, dedicated visual-diffs artifact
- dev:mock script for local fake-provider development
- test:e2e:visual + test:e2e:update-snapshots scripts using cage
- .gitignore: *-snapshots/ (baselines cached in CI, not committed)

.github/workflows/ci.yml

@@ -87,6 +87,12 @@ jobs:
     uses: ./.github/workflows/js-tests.yml
     secrets: inherit
 
+  e2e-desktop:
+    name: Desktop E2E
+    needs: detect
+    if: needs.detect.outputs.python == 'true' || needs.detect.outputs.frontend == 'true'
+    uses: ./.github/workflows/e2e-desktop.yml
+
   docs-site:
     name: Docs Site
     needs: detect
@@ -165,6 +171,7 @@ jobs:
       - tests
       - lint
       - js-tests
+      - e2e-desktop
       - docs-site
       - history-check
       - contributor-check