Add proxy configuration for single-port development

Configure SvelteKit dev server to proxy API requests to Elysia backend,
allowing users to access the application on a single port (5173).

## Changes
- Add proxy config to vite.config.js for /api requests
- Update API client to use relative URLs (/api)
- Add convenience 'dev' script to start both servers
- Update README with single-port instructions
- Add architecture section explaining proxy setup

Benefits:
- Single port to access (5173)
- HMR still works for frontend
- No CORS issues
- Backend runs hidden on port 3001

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

package.json

@@ -4,6 +4,7 @@
   "type": "module",
   "private": true,
   "scripts": {
+    "dev": "bun run src/backend/index.js & cd src/frontend && bun run dev",
     "dev:backend": "bun run src/backend/index.js",
     "dev:frontend": "cd src/frontend && bun run dev",
     "db:generate": "drizzle-kit generate",