[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/rss.less

@@ -1,12 +1,6 @@
 @import (inline) "../../node_modules/normalize.css/normalize.css";
 @import "definitions.less";
-
-.text-size-adjust (@property: 100%) {
-  -webkit-text-size-adjust: @property;
-  -ms-text-size-adjust: @property;
-  -moz-text-size-adjust: @property;
-  text-size-adjust: @property;
-}
+@import "mixins.less";
 
 // Reset padding and margin
 html,