Add the CI workflow and make it run on this project

Add .gitea/workflows/ci.yml: tests, ruff check, ruff format --check and mypy on
every push and pull request, with Python 3.13 on ubuntu-latest.

Two things in the first draft would have failed:

- It installed the dev tools with `pip install -e ".[dev]"`, but the project
  declares them as a dependency group, not an extra. pip only warns and exits 0,
  so pytest, ruff and mypy were missing and the first step died with "No module
  named pytest". It now runs `pip install -e . --group dev`.
- The 10 test cases that use Tk need a display, which a CI runner does not have.
  The workflow now installs xvfb and python3-tk and runs the tests with
  xvfb-run, so the display requirement accepted on RC-006 still holds. No test
  code changes.

Checked locally: the YAML parses to the intended eight steps, the install line
gives all three tools in a clean environment, and the test, lint, format and
type commands pass as written. The Linux display steps have not run yet; they
need a runner.

.gitea/workflows/ci.yml

@@ -0,0 +1,37 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  checks:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.13"
+      - name: Install system packages
+        # The tests open a hidden Tk window, so the runner needs the Tk libraries
+        # and a virtual display (xvfb). sudo is used only where the image has it.
+        env:
+          DEBIAN_FRONTEND: noninteractive
+        run: |
+          SUDO=$(command -v sudo || true)
+          $SUDO apt-get update
+          $SUDO apt-get install -y xvfb python3-tk
+      - name: Install
+        # The dev tools are a dependency group in pyproject.toml, not an extra,
+        # so they need --group dev (pip 25.1 or newer).
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -e . --group dev
+      - name: Tests
+        run: xvfb-run -a python -m pytest
+      - name: Lint
+        run: python -m ruff check src tests
+      - name: Format check
+        run: python -m ruff format --check src tests
+      - name: Types
+        run: python -m mypy src tests