[enh] theme/simple: custom router

Lay the foundation for loading scripts granularly depending on the endpoint it's
on.

Remove vendor specific prefixes as there are now managed by browserslist and
LightningCSS.

Enabled quite a few rules in Biome that don't come in recommended to better
catch issues and improve consistency.

Related:

- https://github.com/searxng/searxng/pull/5073#discussion_r2256037965
- https://github.com/searxng/searxng/pull/5073#discussion_r2256057100

client/simple/src/less/style-rtl.less

@@ -129,13 +129,7 @@
 }
 
 // select HTML element
-@supports (
-  (background-position-x: 100%) and
-    (
-      (appearance: none) or (-webkit-appearance: none) or
-        (-moz-appearance: none)
-    )
-) {
+@supports ((background-position-x: 100%) and ((appearance: none))) {
   select {
     border-width: 0 0 0 2rem;
     background-position-x: -2rem;