Convert admin pages to new Tailwind design pattern

- Replace 'card' class with 'admin-card' (gradient cards, dark mode support)
- Replace 'muted' class with consistent Tailwind muted states
- Standardize page headers with gradient backgrounds and Lucide icons
- Convert all inline styles to Tailwind classes
- Add admin-page CSS helpers (admin-card, admin-filter-bar, admin-section-title)
- Apply pattern to all 42 admin pages (was only 6 before)
- Zero TypeScript errors, production builds successfully

Co-authored-by: openhands <[email protected]>

src/app/admin/articles/new/page.tsx

@@ -1,4 +1,5 @@
 import Link from "next/link";
+import { Newspaper } from "lucide-react";
 import { createArticle } from "@/actions/admin-articles";
 import { ArticleForm } from "@/components/admin/article-form";
 
@@ -11,12 +12,17 @@ export default async function NewArticle({
 
   return (
     <main>
-      <p className="muted">
-        <Link href="/admin/articles">← Articles</Link>
-      </p>
-      <h1>New article</h1>
+      <div className="flex items-center gap-3 mb-6">
+        <div className="w-10 h-10 rounded-xl bg-gradient-to-br from-[var(--color-primary)]/20 to-[var(--color-primary)]/5 grid place-items-center">
+          <Newspaper size={20} className="text-[var(--color-primary)]" />
+        </div>
+        <div>
+          <h1 className="m-0 text-xl font-extrabold text-[var(--color-text)]">New article</h1>
+          <p className="m-0 text-xs text-[var(--color-text-muted)] mt-0.5"><Link href="/admin/articles">← Articles</Link></p>
+        </div>
+      </div>
       {error ? (
-        <p className="muted" style={{ marginBottom: "1rem", color: "#dc2626" }}>
+        <p className="text-xs text-red-600 dark:text-red-400 mb-4">
           Error: {error}
         </p>
       ) : null}