[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/mixins.less

@@ -2,9 +2,6 @@
 
 // Mixins
 .text-size-adjust (@property: 100%) {
-  -webkit-text-size-adjust: @property;
-  -ms-text-size-adjust: @property;
-  -moz-text-size-adjust: @property;
   text-size-adjust: @property;
 }
 
@@ -22,7 +19,6 @@
 
 // disable user selection
 .disable-user-select () {
-  -webkit-touch-callout: none;
   user-select: none;
 }