feat: unify on single repo-root compose file for dev and prod

.forgejo/workflows/frontend.yml

@@ -0,0 +1,20 @@
+# Frontend typecheck + build. Same prerequisites as backend.yml (Runner
+# with the `docker` label, action resolution on the Forgejo instance).
+on:
+  push:
+    branches: [main]
+  pull_request:
+    paths:
+      - "frontend/**"
+      - "docker-compose.yml"
+      - ".forgejo/workflows/frontend.yml"
+
+jobs:
+  build:
+    runs-on: docker
+    container:
+      image: node:22-slim
+    steps:
+      - uses: actions/checkout@v4
+      - run: cd frontend && npm ci
+      - run: cd frontend && npm run build