fix: properly bundle and cache frontend assets

- Build frontend assets natively with Bun to dist/public/ via `src/index.html` entrypoint
- Serve static assets in production with `Cache-Control: public, max-age=31536000, immutable` caching headers
- Preserve native `bun run dev` routing for seamless HMR by retaining development mode conditions in the server router
- Exclude `index.html` from long-term cache headers to allow for production updates

package.json

@@ -4,7 +4,7 @@
   "private": true,
   "scripts": {
     "dev": "bun --hot src/server.tsx",
-    "build": "rm -rf dist && bun build --target=bun ./src/server.tsx --outdir ./dist",
+    "build": "rm -rf dist && bun build --minify --target=browser ./src/index.html --outdir ./dist/public && bun build --target=bun ./src/server.tsx --outdir ./dist",
     "start": "cd dist && DATABASE_PATH=../data/excalidraw.sqlite bun ./server.js",
     "test": "bun test",
     "typecheck": "tsc --noEmit"