fix(webapp): rename the Theme section to Appearance

Matches the account menu's submenu title.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: samejr

apps/webapp/app/routes/account._index/route.tsx

@@ -624,7 +624,7 @@ export default function Page() {
           {showThemeSwitcher && (
             <>
               <div className="mt-8 w-full border-b border-grid-dimmed pb-3">
-                <Header2>Theme</Header2>
+                <Header2>Appearance</Header2>
               </div>
               <div className="flex min-h-16 w-full items-center border-b border-grid-dimmed">
                 <div className="flex w-full items-center justify-between gap-4">