Set vite base to /app/ to support landing-page + app subdirectory site structure

The marketing landing page is served at root (/), and the uni-app SPA
is served at /app/. Setting base: '/app/' ensures all static asset
references (CSS, JS, favicon) are correctly prefixed for the new
subdirectory deployment.

uni-app/vite.config.js

@@ -2,6 +2,7 @@ import { defineConfig } from 'vite'
 import uni from '@dcloudio/vite-plugin-uni'
 
 export default defineConfig({
+  base: '/app/',
   plugins: [uni()],
   server: {
     port: 5173,