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/embed-snippet.tsx

@@ -37,7 +37,7 @@ export function EmbedSnippet({
   const lines = snippet.split('\n').length;
 
   return (
-    <section className="admin-section">
+    <section className="mt-6">
       <div style={{ display: 'flex', alignItems: 'center', gap: '1rem' }}>
         <h2 style={{ flex: 1, margin: 0 }}>{title}</h2>
         <button type="button" className="btn btn-primary" onClick={copy}>