build: set vite base to /duplicate/ for sub-path hosting

Hosted under chess.sethpc.xyz/duplicate/ as a third game-mode card on
blind_chess. With base='/', Vite emitted absolute /assets/... URLs that
would fall through the Caddy /duplicate/* handler and 404 against the
blind_chess Fastify backend. Setting base='/duplicate/' keeps every asset
fetch inside the static handler.

vite.config.ts

@@ -3,5 +3,9 @@ import { svelte } from '@sveltejs/vite-plugin-svelte'
 
 // https://vite.dev/config/
 export default defineConfig({
+  // Hosted as a sub-app under chess.sethpc.xyz/duplicate/ alongside blind_chess.
+  // Setting `base` makes Vite emit asset URLs with the /duplicate/ prefix so they
+  // stay inside the Caddy static handler instead of falling through to blind_chess.
+  base: '/duplicate/',
   plugins: [svelte()],
 })