fix(test): resolve CI failures from the overhaul

- fixture-integrity: probe media via media-engine probeMedia (resolves the
  bundled static ffmpeg) instead of bare system ffprobe, which is ENOENT in CI;
  gate on ffmpegAvailable() like the other media tests
- a11y: regenerate a11y-baseline.json to include the mobile device keys (the
  baseline only had desktop keys, so the mobile a11y scan saw them as new)
- device-visual: tag @visual and exclude it from the PR mobile-smoke gate
  (darwin-only screenshots cannot pass on linux; nightly + update-visual-baselines
  still run it to seed linux goldens)

.github/workflows/ci.yml

@@ -189,7 +189,7 @@ jobs:
       - name: Install Playwright Chromium
         run: pnpm playwright install --with-deps chromium
       - name: Run mobile device specs (Pixel 7)
-        run: pnpm playwright test --project=mobile-chromium --grep @mobile
+        run: pnpm playwright test --project=mobile-chromium --grep @mobile --grep-invert @visual
         env:
           PW_WORKERS: "2"
       - name: Upload report on failure