Add Vite proxy for API requests to backend during development

Proxy /api/* to http://localhost:8080 so dev frontend can call backend API.

frontend/vite.config.ts

@@ -3,5 +3,13 @@ import tailwindcss from '@tailwindcss/vite';
 import { defineConfig } from 'vite';
 
 export default defineConfig({
-  plugins: [tailwindcss(), sveltekit()]
+  plugins: [tailwindcss(), sveltekit()],
+  server: {
+    proxy: {
+      '/api': {
+        target: 'http://localhost:8080',
+        changeOrigin: true
+      }
+    }
+  }
 });