fix: build with relative base URL

snapweb was built using / (absolute path) as base URL.
However, hard-coding an absolute base path prevents serving snapweb under a different path, e.g. exposing snapweb under `/snapcast/` via a reverse-proxy.
To support those kinds of setups, snapweb is built with a relative base URL now.

vite.config.ts

@@ -4,6 +4,7 @@ import { VitePWA } from 'vite-plugin-pwa';
 
 // https://vitejs.dev/config/
 export default defineConfig({
+  base: './',
   plugins: [react(),
   VitePWA({
     registerType: 'autoUpdate',