Set up GitHub Actions CI for automated testing

Implements issue #38 - continuous testing on every PR
- JVM tests run on Windows and macOS
- ComposeApp desktop tests run on Windows and macOS
- Gradle caching for faster builds
- Test reports published as artifacts
- Build status badge available

Author: nikitos4683

.github/workflows/gradle.yml

@@ -0,0 +1,71 @@
+name: Build & Test
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: Test - ${{ matrix.platform }}
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 15
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: windows-latest
+            platform: Windows
+          - os: macos-latest
+            platform: macOS
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+
+    - name: Validate Gradle wrapper
+      uses: gradle/actions/wrapper-validation@v4
+
+    - name: Setup JDK 21
+      uses: actions/setup-java@v4
+      with:
+        java-version: '21'
+        distribution: 'temurin'
+
+    - name: Grant execute permission
+      if: runner.os != 'Windows'
+      run: chmod +x ./gradlew
+
+    - name: Setup Gradle
+      uses: gradle/actions/setup-gradle@v4
+      with:
+        cache-read-only: ${{ github.ref != 'refs/heads/main' }}
+        add-job-summary: on-failure
+
+    - name: Execute JVM tests
+      run: ${{ runner.os == 'Windows' && '.\gradlew.bat' || './gradlew' }} :library:jvmTest --no-daemon --stacktrace
+
+    - name: Execute ComposeApp desktop tests
+      run: ${{ runner.os == 'Windows' && '.\gradlew.bat' || './gradlew' }} :composeApp:desktopTest --no-daemon --stacktrace
+
+    - name: Publish test report
+      if: always()
+      uses: actions/upload-artifact@v4
+      with:
+        name: test-report-${{ matrix.platform }}
+        path: |
+          library/build/test-results/jvmTest/
+          library/build/reports/tests/jvmTest/
+          composeApp/build/test-results/desktopTest/
+          composeApp/build/reports/tests/desktopTest/
+        retention-days: 7
+        if-no-files-found: ignore