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/components/admin/admin-nav-link.tsx

@@ -14,7 +14,12 @@ export function AdminNavLink({ href, label }: { href: string; label: string }) {
     href === "/admin" ? pathname === "/admin" : pathname === href || pathname.startsWith(`${href}/`);
 
   return (
-    <Link href={href} className={`admin-nav-link${active ? " active" : ""}`}>
+    <Link
+      href={href}
+      className={`flex items-center gap-2 px-2.5 py-1.5 rounded-lg text-[#c8cbe0] text-sm font-semibold no-underline transition-[background,color] duration-150 hover:bg-white/7 hover:text-white hover:no-underline ${
+        active ? "bg-[var(--color-primary)] text-[#1a1a2e] shadow-[0_2px_8px_rgba(238,180,37,0.35)]" : ""
+      }`}
+    >
       {label}
     </Link>
   );