Add GitHub Actions CI/CD and Docker Compose

- ci.yml: lint + type-check + build on every push/PR to main; uploads dist artifact
- docker.yml: builds and pushes image to ghcr.io/nmemmert/letter on main push and version tags; uses GHA layer cache
- docker-compose.yml: `app` service runs the production nginx image on :8080; `dev` profile mounts source and runs vite dev server on :5173

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    name: Type-check & build
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+
+      - run: npm ci
+
+      - name: Lint
+        run: npm run lint
+
+      - name: Type-check + build
+        run: npm run build
+
+      - name: Upload dist artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: dist
+          path: dist/
+          retention-days: 7