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/calendar/page.tsx

@@ -62,7 +62,7 @@ export default async function AdminCalendar() {
         Read-only · {campaigns.length} campaign{campaigns.length === 1 ? "" : "s"}.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard label="Campaigns" value={campaigns.length} icon="📅" />
         <StatusCard
           label="Enabled"
@@ -104,7 +104,7 @@ export default async function AdminCalendar() {
                     }}
                   >
                     <h3 style={{ margin: 0 }}>{c.name || `Campaign #${c.id}`}</h3>
-                    <span className={`admin-badge ${enabled ? "ok" : "danger"}`}>
+                    <span className={`inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full ${enabled ? "bg-green-600/20 text-[var(--color-text)]" : "bg-red-500/22 text-[var(--color-text)]"}`}>
                       {enabled ? "enabled" : "disabled"}
                     </span>
                   </div>