Add Vite dev proxy for backend API

Proxy /api requests to localhost:8080 during development.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/vite.config.ts

@@ -15,4 +15,12 @@ export default defineConfig({
       '@': fileURLToPath(new URL('./src', import.meta.url))
     },
   },
+  server: {
+    proxy: {
+      '/api': {
+        target: 'http://localhost:8080',
+        changeOrigin: true,
+      },
+    },
+  },
 })