[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/code.less

@@ -18,11 +18,7 @@
       cursor: default;
 
       &::selection {
-        background: transparent; /* WebKit/Blink Browsers */
-      }
-
-      &::-moz-selection {
-        background: transparent; /* Gecko Browsers */
+        background: transparent;
       }
 
       margin-right: 8px;