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/rooms/page.tsx

@@ -73,7 +73,7 @@ export default async function AdminRooms({
         {total} room{total === 1 ? "" : "s"} total.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard label={q ? "Matching rooms" : "Total rooms"} value={total} icon="🏠" />
         <StatusCard label="Open (this page)" value={openOnPage} state="ok" icon="🔓" />
         <StatusCard label="Users in-room (this page)" value={usersOnPage} icon="👥" />
@@ -110,7 +110,7 @@ export default async function AdminRooms({
                   {r.users} / {r.usersMax}
                 </td>
                 <td>
-                  <span className={`admin-badge ${r.state === "open" ? "ok" : "danger"}`}>
+                  <span className={`inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full ${r.state === "open" ? "bg-green-600/20 text-[var(--color-text)]" : "bg-red-500/22 text-[var(--color-text)]"}`}>
                     {r.state}
                   </span>
                 </td>