[gh-actions] Add rebuild of web UI workflow on push to web-src

.github/workflows/build_htdocs.yml

@@ -0,0 +1,39 @@
+name: Build htdocs
+
+on:
+  push:
+    branches:
+      - master
+    paths:
+      - 'web-src/**'
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install dependencies
+        working-directory: web-src
+        run: npm install
+
+      # Build for production with minification (will update web interface
+      # in "../htdocs")
+      - name: Build htdocs
+        working-directory: web-src
+        run: npm run build
+
+      - name: Count changed files
+        id: count
+        run: |
+          git add htdocs/
+          git diff --numstat --staged > diffstat
+          test -s diffstat || { echo "Warning: Push to web-src did not change htdocs"; exit 1; }
+
+      # The GH action email is from https://github.com/orgs/community/discussions/26560
+      - name: Commit and push updated assets
+        run: |
+          git config --global user.name "github-actions[bot]"
+          git config --global user.email "41898282+github-actions[bot]@users.noreply.github.com"
+          git commit -m "[web] Rebuild web interface"
+          git push