QA: run Windows and macOS regression tests and Windows GUI smoke

.github/workflows/cross-platform-qa.yml

@@ -0,0 +1,56 @@
+name: Cross-platform desktop QA
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  portable-tests:
+    name: Python ${{ matrix.os }} / ${{ matrix.python }}
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 20
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: windows-latest
+            python: "3.12"
+          - os: windows-latest
+            python: "3.13"
+          - os: macos-latest
+            python: "3.12"
+          - os: macos-latest
+            python: "3.13"
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python }}
+          cache: pip
+          cache-dependency-path: requirements.txt
+      - name: Install dependencies
+        run: python -m pip install -r requirements.txt
+      - name: Compile sources
+        run: python -m compileall -q app tests
+      - name: Run platform regression suite
+        run: python -m pytest -q
+      - name: Launch Tkinter desktop window (Windows)
+        if: runner.os == 'Windows'
+        shell: pwsh
+        run: |
+          @'
+          import tkinter as tk
+          from app.gui import ShiftPlannerApp
+          root = tk.Tk()
+          root.withdraw()
+          app = ShiftPlannerApp(root)
+          root.update_idletasks()
+          assert root.title() == "Vagtplanlægning"
+          root.destroy()
+          print("Tkinter window initialised successfully")
+          '@ | python -