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/vpn/page.tsx

@@ -31,7 +31,7 @@ export default async function AdminVpn({
 
   return (
     <main>
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>VPN / Proxy Detection</h2>
         <p className="muted" style={{ marginTop: "-0.4rem" }}>
           Block registrations from detected VPN/proxy IPs at sign-up. Settings
@@ -41,12 +41,12 @@ export default async function AdminVpn({
         </p>
 
         {sp.saved ? (
-          <p className="admin-badge ok" style={{ display: "inline-block" }}>
+          <p className="inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full bg-green-600/20 text-[var(--color-text)]" style={{ display: "inline-block" }}>
             VPN settings saved
           </p>
         ) : null}
 
-        <div className="admin-stats">
+        <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
           <StatusCard
             label="Protection"
             value={enabled ? "On" : "Off"}
@@ -57,7 +57,7 @@ export default async function AdminVpn({
         </div>
       </section>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Settings</h2>
         <form action={saveVpn} className="card">
           <div style={{ marginBottom: "1rem" }}>
@@ -147,7 +147,7 @@ export default async function AdminVpn({
         </form>
       </section>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Raw IP allow / deny list</h2>
         <p className="muted" style={{ marginTop: "-0.4rem" }}>
           VPN detection screens unknown IPs automatically. To explicitly allow