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>

src/frontend/src/lib/api.js

@@ -1,9 +1,10 @@
 import { browser } from '$app/environment';
 
 /**
- * API base URL - change this to match your backend
+ * API base URL - proxied through SvelteKit dev server
+ * In production, this will be relative to the same origin
  */
-const API_BASE = 'http://localhost:3001/api';
+const API_BASE = '/api';
 
 /**
  * Make an API request

src/frontend/vite.config.js

@@ -38,6 +38,14 @@ export default defineConfig({
 		// Disable error overlay to dismiss URI malformed errors
 		hmr: {
 			overlay: false
+		},
+		// Proxy API requests to backend
+		proxy: {
+			'/api': {
+				target: 'http://localhost:3001',
+				changeOrigin: true,
+				secure: false,
+			}
 		}
 	}
 });