ci: force bash as default shell in container jobs

When jobs run with `container:` set, Gitea Actions defaults to
`sh -e` (dash on Ubuntu), so `set -euo pipefail` fails with
"Illegal option -o pipefail". Pinning bash workflow-wide
matches what the runner used pre-container and keeps existing
scripts portable.

.gitea/workflows/e2e.yml

@@ -22,6 +22,11 @@ on:
     branches: [main]
   workflow_dispatch:
 
+# Force bash as the default shell — see ci.yml header.
+defaults:
+  run:
+    shell: bash
+
 jobs:
   e2e:
     name: Playwright vs docker-compose