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/photos/page.tsx

@@ -26,13 +26,13 @@ export default async function AdminPhotos() {
         <code>camera_web</code>.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard label="Photos shown" value={photos.length} icon="📸" />
         <StatusCard label="Rooms" value={uniqueRooms} hint="distinct rooms" icon="🚪" />
         <StatusCard label="Users" value={uniqueUsers} hint="distinct photographers" icon="👤" />
       </div>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Recent photos</h2>
         {photos.length === 0 ? (
           <p className="muted">No photos found.</p>