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/ip/page.tsx

@@ -49,7 +49,7 @@ export default async function AdminIp() {
         Allow or block IP addresses (and ASNs) at the website edge.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard
           label="Whitelisted"
           value={whitelist.length}
@@ -66,7 +66,7 @@ export default async function AdminIp() {
 
       <div className="grid cols-2">
         {/* ── Whitelist ──────────────────── */}
-        <section className="admin-section">
+        <section className="mt-6">
           <h2>Whitelist</h2>
 
           <form action={addWhitelist} className="card" style={{ marginBottom: "1rem" }}>
@@ -120,7 +120,7 @@ export default async function AdminIp() {
         </section>
 
         {/* ── Blacklist ──────────────────── */}
-        <section className="admin-section">
+        <section className="mt-6">
           <h2>Blacklist</h2>
 
           <form action={addBlacklist} className="card" style={{ marginBottom: "1rem" }}>