fix build workflow: add setup-python, reorder steps

add actions/setup-python for fpdf2 dependency (PEP 668 blocks bare
pip install on macOS runners). run bundle build before validation
so keylayout files exist when validator runs.

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

.github/workflows/build.yml

@@ -20,12 +20,20 @@ jobs:
       - name: checkout
         uses: actions/checkout@v4
 
-      - name: validate layouts
-        run: python3 scripts/validate_layouts.py
+      - name: setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - name: install Python dependencies
+        run: pip install fpdf2
 
       - name: build bundle
         run: bash scripts/build-bundle.sh
 
+      - name: validate layouts
+        run: python3 scripts/validate_layouts.py
+
       - name: create DMG
         run: bash scripts/create-dmg.sh