🎨 style: remove ambient grid background from app shells

Remove the decorative grid background layer from both the main app shell
and authentication shell to create a cleaner, more minimal UI appearance.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

components/shell/app-shell.tsx

@@ -518,7 +518,6 @@ export function AppShell({
 
   return (
     <div className="app-surface">
-      <div className="ambient-grid" aria-hidden="true" />
       <div className="noise-layer" aria-hidden="true" />
 
       <div