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/history/page.tsx

@@ -95,7 +95,7 @@ export default async function AdminRadioHistoryPage() {
           </p>
         </div>
       ) : (
-        <div className="admin-stats">
+        <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
           <StatusCard label="Shows logged" value={history.length} icon="🎙️" />
           <StatusCard
             label="On air now"
@@ -113,7 +113,7 @@ export default async function AdminRadioHistoryPage() {
       ) : null}
 
       {history.length > 0 ? (
-        <section className="admin-section">
+        <section className="mt-6">
           <h2>Show log ({history.length})</h2>
           <table>
             <thead>
@@ -139,7 +139,7 @@ export default async function AdminRadioHistoryPage() {
                     {h.endedAt ? (
                       <span className="muted">{formatDate(h.endedAt)}</span>
                     ) : (
-                      <span className="admin-badge ok">on air</span>
+                      <span className="inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full bg-green-600/20 text-[var(--color-text)]">on air</span>
                     )}
                   </td>
                   <td className="muted">{formatDuration(h.startedAt, h.endedAt)}</td>