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/users/page.tsx

@@ -37,7 +37,7 @@ export default async function AdminUsers({
         {q ? `Search results for “${q}”.` : "Most recent accounts first."}
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard label="Total users" value={total} icon="👥" />
         <StatusCard label="On this page" value={users.length} icon="📄" />
         <StatusCard
@@ -63,7 +63,7 @@ export default async function AdminUsers({
         </button>
       </form>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Accounts ({total})</h2>
         {users.length === 0 ? (
           <p className="muted">No users match this search.</p>
@@ -90,7 +90,7 @@ export default async function AdminUsers({
                     <td>{u.rank}</td>
                     <td>{u.credits}</td>
                     <td>
-                      <span className={`admin-badge ${u.online === "1" ? "ok" : ""}`}>
+                      <span className={`inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full ${u.online === "1" ? "bg-green-600/20 text-[var(--color-text)]" : "bg-[var(--color-primary)]/18 text-[var(--color-text)]"}`}>
                         {u.online === "1" ? "online" : "offline"}
                       </span>
                     </td>