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>

docker-compose.yml

@@ -0,0 +1,21 @@
+services:
+  app:
+    build: .
+    image: ghcr.io/nmemmert/letter:main
+    ports:
+      - "8080:80"
+    restart: unless-stopped
+
+  dev:
+    image: node:20-alpine
+    working_dir: /app
+    volumes:
+      - .:/app
+      - node_modules:/app/node_modules
+    ports:
+      - "5173:5173"
+    command: sh -c "npm install && npm run dev -- --host 0.0.0.0"
+    profiles: [dev]
+
+volumes:
+  node_modules: