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/teams/page.tsx

@@ -40,7 +40,7 @@ export default async function AdminTeams() {
         Staff rank definitions shown on the public team page.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard label="Team ranks" value={teams.length} icon="🛡️" />
         <StatusCard
           label="Visible"
@@ -84,7 +84,7 @@ export default async function AdminTeams() {
         </form>
       </div>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Ranks ({teams.length})</h2>
         {teams.length === 0 ? (
           <p className="muted">No teams yet.</p>
@@ -131,7 +131,7 @@ export default async function AdminTeams() {
                       </span>
                     </td>
                     <td>
-                      <span className={`admin-badge ${t.hiddenRank ? "" : "ok"}`}>
+                      <span className={`inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full ${t.hiddenRank ? "bg-[var(--color-primary)]/18 text-[var(--color-text)]" : "bg-green-600/20 text-[var(--color-text)]"}`}>
                         {t.hiddenRank ? "hidden" : "visible"}
                       </span>
                     </td>