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/monitoring/page.tsx

@@ -224,7 +224,7 @@ export default async function AdminRadioMonitoringPage() {
         <Link href="/admin/radio/history">History</Link>
       </nav>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Radio Monitoring</h2>
         <p className="muted" style={{ marginTop: '-0.4rem' }}>
           Read-only live probe of the configured radio endpoints
@@ -234,7 +234,7 @@ export default async function AdminRadioMonitoringPage() {
           <Link href="/admin/radio/settings">Settings</Link>.
         </p>
 
-        <div className="admin-stats">
+        <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
           <StatusCard
             label="Stream"
             value={
@@ -261,14 +261,14 @@ export default async function AdminRadioMonitoringPage() {
         </div>
       </section>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Endpoint diagnostics</h2>
         <p className="muted" style={{ marginTop: '-0.4rem' }}>
-          <span className="admin-badge ok">OK</span> reachable ·{' '}
-          <span className="admin-badge">WARN</span> not configured ·{' '}
-          <span className="admin-badge danger">ERROR</span> fetch failed / timed out.
+          <span className="inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full bg-green-600/20 text-[var(--color-text)]">OK</span> reachable ·{' '}
+          <span className="inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full bg-[var(--color-primary)]/18 text-[var(--color-text)]">WARN</span> not configured ·{' '}
+          <span className="inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full bg-red-500/22 text-[var(--color-text)]">ERROR</span> fetch failed / timed out.
         </p>
-        <div className="admin-diag">
+        <div className="flex flex-col">
           {endpoints.map((ep) => {
             let state: 'ok' | 'warn' | 'danger';
             let detail: string;