ci: add unit-test workflow

Runs Vitest on every push and PR to main. Catches regressions before
GitHub Pages auto-deploys main.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: shainova

.github/workflows/test.yml

@@ -0,0 +1,19 @@
+name: tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  vitest:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+      - run: npm ci
+      - run: npm test