Add asset pipeline and professionalize theme

Introduce a Node build system for minified CSS/JS with content hashes,
add PWA support, i18n helpers, accessibility improvements, and more
translatable strings. Move theme configuration from bookmarks to
localStorage, add Playwright tests, and set up CI workflows.

.github/workflows/ci.yml

@@ -0,0 +1,42 @@
+name: CI
+
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+    branches: [main, master]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: shaarli-pro
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+          cache-dependency-path: shaarli-pro/package-lock.json
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Build theme assets
+        run: npm run build
+
+      - name: Verify JavaScript syntax
+        run: |
+          for f in js/*.js js/modules/*.js dist/*.js; do
+            node --check "$f"
+          done
+
+      - name: Ensure dist/ is up-to-date
+        run: |
+          git diff --exit-code dist/ || (
+            echo "dist/ is out of date. Run 'npm run build' and commit the changes." && exit 1
+          )