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/vouchers/page.tsx

@@ -61,7 +61,7 @@ export default async function AdminVouchers() {
         use a given voucher once, up to its max uses.
       </p>
 
-      <div className="admin-stats">
+      <div className="grid grid-cols-[repeat(auto-fit,minmax(180px,1fr))] gap-3.5 mb-6">
         <StatusCard label="Vouchers" value={vouchers.length} icon="🎟️" />
         <StatusCard
           label="Active"
@@ -106,7 +106,7 @@ export default async function AdminVouchers() {
         <button type="submit" className="btn btn-primary">Create voucher</button>
       </form>
 
-      <section className="admin-section">
+      <section className="mt-6">
         <h2>Vouchers ({vouchers.length})</h2>
         <div className="card">
           <table>
@@ -136,11 +136,11 @@ export default async function AdminVouchers() {
                     </td>
                     <td>
                       {expired ? (
-                        <span className="admin-badge danger">Expired</span>
+                        <span className="inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full bg-red-500/22 text-[var(--color-text)]">Expired</span>
                       ) : usedUp ? (
-                        <span className="admin-badge">Used up</span>
+                        <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)]">Used up</span>
                       ) : (
-                        <span className="admin-badge ok">Active</span>
+                        <span className="inline-block text-[0.72rem] font-bold px-2 py-0.5 rounded-full bg-green-600/20 text-[var(--color-text)]">Active</span>
                       )}
                     </td>
                     <td className="muted">{fmtDate(v.expiresAt)}</td>