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/radio/api-keys/page.tsx

@@ -74,7 +74,7 @@ export default async function AdminRadioApiKeysPage() {
         <Link href="/admin/radio/monitoring">Monitoring</Link>
       </nav>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h1>Radio API Keys</h1>
         <p className="muted" style={{ marginTop: "-0.4rem" }}>
           API keys for external radio integrations (<code>radio_api_keys</code>). Keys are generated
@@ -88,7 +88,7 @@ export default async function AdminRadioApiKeysPage() {
             </p>
           </div>
         ) : (
-          <div className="admin-stats">
+          <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
             <StatusCard label="Total keys" value={total} icon="🔑" />
             <StatusCard
               label="Active"
@@ -106,7 +106,7 @@ export default async function AdminRadioApiKeysPage() {
         )}
       </section>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Generate key</h2>
         <p className="muted" style={{ marginTop: "-0.4rem" }}>
           The secret key is generated automatically with cryptographic randomness — you do not enter
@@ -161,7 +161,7 @@ export default async function AdminRadioApiKeysPage() {
         </form>
       </section>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Existing keys ({total})</h2>
         {total === 0 ? (
           <p className="muted">No API keys yet — generate one above.</p>
@@ -194,7 +194,7 @@ export default async function AdminRadioApiKeysPage() {
                     <td className="muted">{formatDate(k.lastUsedAt)}</td>
                     <td className="muted">{formatDate(k.expiresAt)}</td>
                     <td>
-                      <span className={`admin-badge ${k.isActive ? "ok" : "danger"}`}>
+                      <span className={`inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full ${k.isActive ? "bg-green-600/20 text-[var(--color-text)]" : "bg-red-500/22 text-[var(--color-text)]"}`}>
                         {k.isActive ? "active" : "inactive"}
                       </span>
                     </td>