ci: add tests workflow + gate releases on tests passing

- .gitea/workflows/tests.yml: run `unittest discover` on push + pull_request.
  `core` job (stdlib install, GUI tests skip) is bulletproof; `gui-smoke` job
  installs the GUI extra + offscreen Qt libs and runs the suite headless.
- release.yml: add a `test` job and `release: needs: test` so a push to main
  can't publish if the tests fail.

No version bump — CI config only; nothing in the shipped app changed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -11,7 +11,20 @@ on:
     branches: [main]
 
 jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+      - name: Install (core only)
+        run: python -m pip install -e .
+      - name: Run tests
+        run: python -m unittest discover -s tests -v
+
   release:
+    needs: test          # don't publish a release if the tests fail
     runs-on: ubuntu-latest
     steps:
       - name: Checkout