ci: add unit tests workflow

Add GitHub Actions workflow to run Vitest unit tests:
- Runs on push to main/develop and on PRs
- Executes all 292 unit tests
- Generates coverage report on PRs
- Uploads coverage artifacts for review

This provides fast feedback (~20s) compared to E2E tests.

Author: claude

.github/workflows/unit-tests.yml

@@ -0,0 +1,41 @@
+name: Unit Tests
+
+on:
+  push:
+    branches: [main, develop]
+  pull_request:
+    branches: [main, develop]
+
+jobs:
+  test:
+    name: Run Unit Tests
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: pnpm/action-setup@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 18
+          cache: "pnpm"
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Run unit tests
+        run: pnpm test
+
+      - name: Run unit tests with coverage
+        run: pnpm run test:coverage
+        if: github.event_name == 'pull_request'
+
+      - name: Upload coverage reports
+        uses: actions/upload-artifact@v4
+        if: github.event_name == 'pull_request'
+        with:
+          name: coverage-report
+          path: coverage/
+          retention-days: 7