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/settings/page.tsx

@@ -21,12 +21,12 @@ export default async function AdminSettings() {
         Key/value config read at runtime via <code>siteSettings.get()</code>. Saving busts the cache.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard label="Settings" value={settings.length} icon="🗄️" />
         <StatusCard label="With comment" value={documented} icon="📝" />
       </div>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Add / overwrite</h2>
         <form action={createSetting} className="card">
           <div style={{ display: "flex", gap: "0.5rem", flexWrap: "wrap" }}>
@@ -40,9 +40,9 @@ export default async function AdminSettings() {
         </form>
       </section>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>
-          Settings <span className="admin-badge">{settings.length}</span>
+          Settings <span className="inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full bg-[var(--color-primary)]/18 text-[var(--color-text)]">{settings.length}</span>
         </h2>
         {settings.length === 0 ? (
           <p className="muted">No settings yet.</p>