ci: add JaCoCo coverage gate and Gitea CI pipeline

- Add JaCoCo maven plugin (0.8.11) with prepare-agent, report, and check goals
- Set 50% bundle-level line coverage gate (to be raised as test coverage grows)
- Exclude UI classes (App, Controller, ShapesView, etc.) from coverage checks
  since they require a display and cannot be unit-tested
- Add Gitea Actions workflow: checkout → JDK 16 → mvn verify → upload report

Closes #14

.gitea/workflows/ci.yaml

@@ -0,0 +1,30 @@
+name: CI
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  build-and-test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 16
+        uses: actions/setup-java@v4
+        with:
+          java-version: '16'
+          distribution: 'temurin'
+
+      - name: Build and test with coverage
+        run: mvn verify --batch-mode
+
+      - name: Upload coverage report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: jacoco-report
+          path: target/site/jacoco/