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/bans/page.tsx

@@ -32,7 +32,7 @@ export default async function AdminBans() {
         Active bans only (expiry in the future) · newest first.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard
           label="Active bans"
           value={bans.length}
@@ -66,7 +66,7 @@ export default async function AdminBans() {
         </div>
       </form>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Active bans ({bans.length})</h2>
         {bans.length === 0 ? (
           <p className="muted">No active bans.</p>
@@ -87,12 +87,12 @@ export default async function AdminBans() {
                   <tr key={b.id}>
                     <td>{b.userId}</td>
                     <td>
-                      <span className="admin-badge">{b.type}</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)]">{b.type}</span>
                     </td>
                     <td>{b.banReason || "—"}</td>
                     <td>
                       {b.banExpire === 0 ? (
-                        <span className="admin-badge danger">permanent</span>
+                        <span className="inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full bg-red-500/22 text-[var(--color-text)]">permanent</span>
                       ) : (
                         <span className="muted" style={{ whiteSpace: "nowrap" }}>
                           {fromUnix(b.banExpire)}