fix(ci): add TZ=Europe/Berlin to frontend test step

date-buckets.spec.ts midnight tests pass timezone-aware dates (+02:00)
which are 22:00 UTC the prior day; setHours(0,0,0,0) uses local TZ.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -39,6 +39,8 @@ jobs:
       - name: Run unit and component tests
         run: npm test
         working-directory: frontend
+        env:
+          TZ: Europe/Berlin
 
       - name: Build frontend
         run: npm run build