[fix] themes: module resolution (#5694)

Vite should not pregen static paths on build, but rather compute the relative path at runtime.

- https://vite.dev/config/shared-options#base

Closes: https://github.com/searxng/searxng/issues/5561

client/simple/vite.config.ts

@@ -34,7 +34,7 @@ const svg2svg_favicon_opts: Config = {
 };
 
 export default {
-  base: "/static/themes/simple/",
+  base: "./",
   publicDir: "static/",
 
   build: {