Unfucks /datum/browse code (#89994)

What it was doing was by and large fine, HOW it was doing it SUCKED
I've cleaned it up and the nearby code some, notable hits include:
- random if check in secrets ui that was totally unused
- proc called add that actually set
- lists not defined as such
- stupid var names
- proc args which did nothing
- code which did nothing
- oververbose code
- proc/var names with no spacing at all

Note: This might have changed behavior accidentally, I've done my best
to test but we'll need to look out for issue reports in coming days.

I was working on bitflag code and saw red, now it's 2 hours later.

🆑
code: Brought browser code up to standard with the rest of the codebase
admin: Hey lads, I cleaned up how non TGUI windows work on the backend,
please let me know if anything is broken! PING ME MOTHERFUCKER
/🆑

code/modules/error_handler/error_viewer.dm

@@ -27,7 +27,7 @@ GLOBAL_DATUM(error_cache, /datum/error_viewer/error_cache)
 /datum/error_viewer/proc/browse_to(client/user, html)
 	var/datum/browser/browser = new(user.mob, "error_viewer", null, 600, 400)
 	browser.set_content(html)
-	browser.add_head_content({"
+	browser.set_head_content({"
 	<style>
 	.runtime
 	{