refactor: convert admin panel to Tailwind CSS

- Convert admin layout, sidebar, and navigation to Tailwind classes
- Convert dashboard components (StatusCard, DiagnosticRow, etc.) to Tailwind
- Convert all ~48 admin page files from admin CSS classes to Tailwind
- Remove unused admin CSS from globals.css (973 → 712 lines)
- Convert developers page badges to Tailwind
- Remove <style jsx> block from OnlineUsersWidget

src/app/admin/ads/page.tsx

@@ -32,12 +32,12 @@ export default async function AdminAds() {
         across the site.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard label="Ads" value={ads.length} icon="🖼️" />
         <StatusCard label="Latest" value={latest} hint="most recent ad" icon="🕒" />
       </div>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>All ads</h2>
         {ads.length === 0 ? (
           <p className="muted">No advertisements yet.</p>