fix: set vite base to /build/ so asset paths match the actual serve location

Assets built to public/build/assets/ but index.html referenced /assets/
(no /build/ prefix). Nginx couldn't find them, fell through to Symfony's
catch-all SPA route, which served HTML in place of JS — Vue never loaded.

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

frontend/vite.config.ts

@@ -9,6 +9,7 @@ export default defineConfig({
       '@': resolve(__dirname, 'src'),
     },
   },
+  base: '/build/',
   build: {
     outDir: '../public/build',
     emptyOutDir: true,