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/autodj/page.tsx

@@ -76,7 +76,7 @@ export default async function AdminRadioAutoDjPage() {
         <Link href="/admin/radio/autodj">AutoDJ</Link>
       </nav>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Radio AutoDJ</h2>
         <p className="muted" style={{ marginTop: '-0.4rem' }}>
           Fallback playlist (<code>radio_auto_dj_playlist</code>) the radio rotates
@@ -90,7 +90,7 @@ export default async function AdminRadioAutoDjPage() {
             </p>
           </div>
         ) : (
-          <div className="admin-stats">
+          <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
             <StatusCard label="Tracks" value={tracks.length} icon="🎵" />
             <StatusCard
               label="Active"
@@ -108,7 +108,7 @@ export default async function AdminRadioAutoDjPage() {
         )}
       </section>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Add track</h2>
         <form action={createTrack} className="card">
           <div className="grid cols-2">
@@ -186,7 +186,7 @@ export default async function AdminRadioAutoDjPage() {
         </form>
       </section>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Playlist ({tracks.length})</h2>
         {tracks.length === 0 ? (
           <p className="muted">No AutoDJ tracks yet — add one above.</p>
@@ -232,7 +232,7 @@ export default async function AdminRadioAutoDjPage() {
                     <td className="muted">{t.playCount}</td>
                     <td className="muted">{formatDate(t.lastPlayedAt)}</td>
                     <td>
-                      <span className={`admin-badge ${t.isActive ? 'ok' : 'danger'}`}>
+                      <span className={`inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full ${t.isActive ? 'bg-green-600/20 text-[var(--color-text)]' : 'bg-red-500/22 text-[var(--color-text)]'}`}>
                         {t.isActive ? 'active' : 'inactive'}
                       </span>
                     </td>