feat: replace localStorage with Bun/Hono API backend

- server.ts: Hono server with basic auth, GET/PUT/DELETE /api/* endpoints
- defaults.json: extracted board defaults from index.html
- Dockerfile: containerized for Coolify deployment
- index.html: all state-layer rewritten from localStorage to fetch API

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

Dockerfile

@@ -0,0 +1,8 @@
+FROM oven/bun:1
+WORKDIR /app
+COPY package.json ./
+RUN bun install --production
+COPY server.ts bootstrap.ts defaults.json index.html ./
+COPY assets/ ./assets/
+EXPOSE 3000
+CMD ["bun", "run", "server.ts"]