Add Mylder brand design system with warm amber accent

- Implement comprehensive design system with OKLCH colors
- Add warm stone neutrals replacing cold zinc grays
- Add amber brand color for CTAs and accents
- Update typography to Plus Jakarta Sans + JetBrains Mono
- Add brand variants to Button and Badge components
- Add utility classes: glass, shadow-brand, text-gradient-brand
- Update all pages to use semantic design tokens
- Add design system documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/app/(auth)/signup/page.tsx

@@ -57,11 +57,11 @@ export default function SignupPage() {
   }
 
   return (
-    <div className="min-h-screen flex items-center justify-center bg-zinc-50 dark:bg-zinc-950 px-4">
+    <div className="min-h-screen flex items-center justify-center bg-background px-4">
       <Card className="w-full max-w-md">
         <CardHeader className="text-center">
           <Link href="/" className="inline-flex items-center justify-center gap-2 mb-4">
-            <Bot className="h-8 w-8 text-primary" />
+            <Bot className="h-8 w-8 text-brand" />
             <span className="text-xl font-bold">Mylder</span>
           </Link>
           <CardTitle>Create your account</CardTitle>
@@ -118,11 +118,11 @@ export default function SignupPage() {
               />
             </div>
             {error && (
-              <p className="text-sm text-red-500">{error}</p>
+              <p className="text-sm text-destructive">{error}</p>
             )}
           </CardContent>
           <CardFooter className="flex flex-col gap-4">
-            <Button type="submit" className="w-full" disabled={isLoading}>
+            <Button type="submit" variant="brand" className="w-full" disabled={isLoading}>
               {isLoading ? (
                 <>
                   <Loader2 className="mr-2 h-4 w-4 animate-spin" />
@@ -132,9 +132,9 @@ export default function SignupPage() {
                 'Create Account'
               )}
             </Button>
-            <p className="text-sm text-center text-zinc-500">
+            <p className="text-sm text-center text-muted-foreground">
               Already have an account?{' '}
-              <Link href="/login" className="text-primary hover:underline">
+              <Link href="/login" className="text-brand hover:underline">
                 Log in
               </Link>
             </p>