🎨 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/auth/auth-shell.tsx

@@ -10,7 +10,6 @@ type AuthShellProps = {
 export function AuthShell({ title, subtitle, children, footer }: AuthShellProps) {
   return (
     <div className="auth-page">
-      <div className="ambient-grid" aria-hidden="true" />
       <div className="noise-layer" aria-hidden="true" />
 
       <div className="relative z-10 mx-auto flex min-h-screen w-full max-w-5xl flex-col justify-center gap-5 px-4 py-6 sm:gap-8 sm:py-10 md:px-8 lg:flex-row lg:items-center">

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