ci: add GitHub Actions workflow that runs the unittest suite (closes #13)

There was no CI on this repository — 137 unit tests in tests/ were only
ever run when a developer remembered to run them locally. A regression
that broke CLI parity, exclusion rules, exporter output, alias
inference, or search filtering could land on master with no gate.

New workflow `.github/workflows/tests.yml`:
- Triggers on every push to master and every pull request.
- Single ubuntu-latest runner, Python 3.12.
- Installs only what the tests need (flask, fpdf2). pywebview from
  requirements.txt is the desktop-launcher dep and pulls GTK / Qt
  system packages — out of scope for the unittest suite, so it is
  deliberately omitted from the CI install. The unittest suite
  imports neither.
- Runs `python -m unittest discover tests -v`.

Local sanity-check with the same command on Python 3.12: 137/137 OK.

Author: timon0305

.github/workflows/tests.yml

@@ -0,0 +1,30 @@
+name: Tests
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+jobs:
+  unittest:
+    name: Unit tests
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - name: Install runtime + test dependencies
+        # Only what the tests actually exercise. `pywebview` from requirements.txt
+        # is the desktop-launcher dep and pulls GTK / Qt system packages on Linux
+        # — out of scope for the unittest suite, so it's deliberately omitted here.
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install 'flask>=3.0' 'fpdf2>=2.7'
+
+      - name: Run unittest suite
+        run: python -m unittest discover tests -v