ci: reuse host-playwright browser cache instead of re-downloading chromium

Point PLAYWRIGHT_BROWSERS_PATH at the persistent /opt/ms-playwright dir on
the host runner so 'playwright install chromium' is an instant no-op after
the first run (was ~100s CDN download per job).

.gitea/workflows/ci.yaml

@@ -8,6 +8,12 @@ on:
     branches: [main, master]
   workflow_dispatch:
 
+# Reuse the Playwright browsers that ship with the host runner (snapped to
+# the root HOME cache instead of a fresh per-job HOME) so `playwright install`
+# is a near-instant no-op instead of a ~100s CDN download on every run.
+env:
+  PLAYWRIGHT_BROWSERS_PATH: /opt/ms-playwright
+
 jobs:
   # ─────────────────────────────────────────────
   # Lint, typecheck & unit tests