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>

.gitignore

@@ -2,4 +2,9 @@ node_modules/
 .DS_Store
 *.log
 backup-*.html
+bun.lock
+extract-defaults.mjs
+transform.mjs
+index.html.bak
+data/