Chore: add CI workflow for lint, test, build

.github/workflows/ci.yml

@@ -0,0 +1,31 @@
+name: CI
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+# Cancel outdated runs on the same branch to save runner time and energy.
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  checks:
+    runs-on: self-hosted
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+
+      # npm ci installs exactly what package-lock.json says, so CI matches
+      # what we run locally.
+      - run: npm ci
+      - run: npm run lint
+      - run: npm run format:check
+      - run: npm test
+      - run: npm run build

frontend/package.json

@@ -8,7 +8,7 @@
     "build": "tsc -b && vite build",
     "lint": "eslint .",
     "preview": "vite preview",
-    "test": "vitest"
+    "test": "vitest --passWithNoTests"
   },
   "dependencies": {
     "leaflet": "^1.9.4",