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/embed/page.tsx

@@ -53,7 +53,7 @@ export default async function AdminRadioEmbedPage() {
         <Link href="/admin/radio/history">History</Link>
       </nav>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h1>Radio Embed</h1>
         <p className="muted" style={{ marginTop: '-0.4rem' }}>
           Copy-ready embed code for the live stream. Read-only — nothing here
@@ -62,7 +62,7 @@ export default async function AdminRadioEmbedPage() {
           <code>radio_stream_url</code>).
         </p>
 
-        <div className="admin-stats">
+        <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
           <StatusCard
             label="Stream URL"
             value={hasStream ? 'Configured' : 'Missing'}
@@ -80,7 +80,7 @@ export default async function AdminRadioEmbedPage() {
       </section>
 
       {!hasStream ? (
-        <section className="admin-section">
+        <section className="mt-6">
           <h2>No stream URL configured</h2>
           <p className="muted">
             There is no <code>radio_stream_url</code> set, so there is nothing to
@@ -98,7 +98,7 @@ export default async function AdminRadioEmbedPage() {
         </section>
       ) : (
         <>
-          <section className="admin-section">
+          <section className="mt-6">
             <h2>Live preview</h2>
             <p className="muted" style={{ marginTop: '-0.4rem' }}>
               This is the bare <code>&lt;audio&gt;</code> player as visitors will