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/maintenance/page.tsx

@@ -41,7 +41,7 @@ export default async function AdminMaintenance() {
         <code> website_settings</code> and read live by the login flow.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard
           label="Maintenance"
           value={enabled ? "ENABLED" : "Disabled"}
@@ -64,10 +64,10 @@ export default async function AdminMaintenance() {
       </div>
 
       {/* ── Current state ─────────────────────────────────────── */}
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Current state</h2>
         <div className="card" style={{ display: "flex", alignItems: "center", gap: "0.75rem" }}>
-          <span className={`admin-badge ${enabled ? "danger" : "ok"}`}>
+          <span className={`inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full ${enabled ? "bg-red-500/22 text-[var(--color-text)]" : "bg-green-600/20 text-[var(--color-text)]"}`}>
             {enabled ? "ENABLED" : "disabled"}
           </span>
           <div>
@@ -84,7 +84,7 @@ export default async function AdminMaintenance() {
       </section>
 
       {/* ── Edit form ─────────────────────────────────────────── */}
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Settings</h2>
         <form action={saveMaintenance} className="card">
           <label