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/navigation/page.tsx

@@ -29,7 +29,7 @@ type PublicRow = {
 // Reusable yes/no pill so emulator boolean-ish "1"/"0" columns read consistently.
 function YesNo({ on }: { on: boolean }) {
   return (
-    <span className={`admin-badge ${on ? "ok" : "danger"}`}>{on ? "Yes" : "No"}</span>
+    <span className={`inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full ${on ? "bg-green-600/20 text-[var(--color-text)]" : "bg-red-500/22 text-[var(--color-text)]"}`}>{on ? "Yes" : "No"}</span>
   );
 }
 
@@ -108,7 +108,7 @@ export default async function AdminNavigation() {
         emulator tooling, not here.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard label="Flat categories" value={flatcats.length} icon="📂" />
         <StatusCard
           label="Public (flatcats)"
@@ -126,7 +126,7 @@ export default async function AdminNavigation() {
       </div>
 
       {/* ── navigator_flatcats ───────────────────────────────── */}
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Flat categories</h2>
         <p className="muted" style={{ marginTop: "-0.4rem" }}>
           <code>navigator_flatcats</code> · {flatcats.length} rows
@@ -174,7 +174,7 @@ export default async function AdminNavigation() {
       </section>
 
       {/* ── navigator_publics ────────────────────────────────── */}
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Public rooms</h2>
         <p className="muted" style={{ marginTop: "-0.4rem" }}>
           <code>navigator_publics</code> · {publics.length} rows