build: use pnpm official Docker image and update CI/Docker setup (#639)

- Replace docker-compose.yml (node:lts) with compose.yaml (ghcr.io/pnpm/pnpm:11.3.0)
  - Bump pnpm to 11.3.0 and add --frozen-lockfile in CI
  - Add pnpm-workspace.yaml to allow esbuild and sharp builds
  - Update .gitignore for pnpm store and compose override file

compose.yaml

@@ -0,0 +1,21 @@
+services:
+  app:
+    image: ghcr.io/pnpm/pnpm:11.3.0
+    ports:
+      - 4321:4321
+    working_dir: /app
+    command: >
+      bash -c "
+      pnpm runtime set node 24 -g
+      && pnpm install --frozen-lockfile
+      && pnpm prune
+      && pnpm run dev --host 0.0.0.0"
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:4321"]
+      interval: 10s
+      timeout: 5s
+      retries: 10
+      start_period: 60s
+    volumes:
+      - ./:/app
+      - /app/node_modules