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/radio/banners/page.tsx

@@ -72,7 +72,7 @@ export default async function AdminRadioBannersPage() {
           </p>
         </div>
       ) : (
-        <div className="admin-stats">
+        <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
           <StatusCard label="Banners" value={banners.length} icon="🖼️" />
           <StatusCard
             label="Active"
@@ -89,7 +89,7 @@ export default async function AdminRadioBannersPage() {
         </div>
       )}
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Add banner</h2>
         <form action={createRadioBanner} className="card">
           <div className="grid cols-2">
@@ -148,7 +148,7 @@ export default async function AdminRadioBannersPage() {
         </form>
       </section>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Existing banners ({banners.length})</h2>
         {banners.length === 0 ? (
           <p className="muted">No banners yet — add one above.</p>
@@ -170,7 +170,7 @@ export default async function AdminRadioBannersPage() {
                     style={{ display: 'inline-flex', gap: '0.5rem', alignItems: 'center' }}
                   >
                     by user {String(b.userId)} · {formatDate(b.createdAt)} ·{' '}
-                    <span className={`admin-badge ${b.isActive ? 'ok' : 'danger'}`}>
+                    <span className={`inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full ${b.isActive ? 'bg-green-600/20 text-[var(--color-text)]' : 'bg-red-500/22 text-[var(--color-text)]'}`}>
                       {b.isActive ? 'active' : 'hidden'}
                     </span>
                   </span>