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/applications/page.tsx

@@ -50,7 +50,7 @@ export default async function AdminApplications() {
         Pending applications from players requesting a staff rank.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard
           label="Pending applications"
           value={applications.length}
@@ -59,7 +59,7 @@ export default async function AdminApplications() {
         />
       </div>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Applications ({applications.length})</h2>
         {applications.length === 0 ? (
           <p className="muted">No pending applications.</p>
@@ -84,7 +84,7 @@ export default async function AdminApplications() {
                         <>User #{a.userId}</>
                       )}
                     </h3>
-                    <span className="admin-badge">Rank {a.rankId}</span>
+                    <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)]">Rank {a.rankId}</span>
                   </div>
                   <p className="muted" style={{ marginTop: 0 }}>
                     {formatDate(a.createdAt)}