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>

package.json

@@ -0,0 +1,5 @@
+{
+  "dependencies": {
+    "hono": "^4"
+  }
+}