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/ads/new/page.tsx

@@ -1,14 +1,20 @@
 import Link from "next/link";
+import { Megaphone } from "lucide-react";
 import { createAd } from "@/actions/admin-ads";
 
 export default function NewAd() {
   return (
     <main>
-      <p className="muted">
-        <Link href="/admin/ads">← Advertisements</Link>
-      </p>
-      <h1>New advertisement</h1>
-      <form action={createAd} className="card" style={{ display: "grid", gap: "0.6rem" }}>
+      <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">
+          <Megaphone size={20} className="text-[var(--color-primary)]" />
+        </div>
+        <div>
+          <h1 className="m-0 text-xl font-extrabold text-[var(--color-text)]">New advertisement</h1>
+          <p className="m-0 text-xs text-[var(--color-text-muted)] mt-0.5"><Link href="/admin/ads">← Advertisements</Link></p>
+        </div>
+      </div>
+      <form action={createAd} className="admin-card">
         <input name="image" placeholder="Image URL" required maxLength={255} />
         <button type="submit" className="btn btn-primary">
           Create