refactor(frontend): audit fixes - accessibility, colors, and UX

Accessibility:
- Add focus-visible indicators to all interactive elements (buttons, inputs)
- Create accessible CreateUserModal to replace window.prompt()
- Add aria-labels to icon buttons for screen readers

Colors & Theming:
- Move primary color to CSS variable (--primary)
- Remove hard-coded #3b82f6 from tailwind config
- Clean up color consistency across theme

UX:
- Replace prompt-based user creation with proper modal form
- Implement inline field validation with error messages
- Add loading state during form submission
- Improve visual hierarchy and spacing

Performance:
- Remove bootstrap-icons dependency (duplicate with lucide-react)

Design:
- Reduce backdrop-blur overuse (remove from overlay scrim, StatCard)
- Improve AdminOverlay responsive design

frontend/components/StatCard.tsx

@@ -9,7 +9,7 @@ interface StatCardProps {
 
 export default function StatCard({ label, value, icon: Icon }: StatCardProps) {
   return (
-    <div className="flex justify-between items-center gap-2 p-3.5 bg-slate-900/50 backdrop-blur-md border border-slate-800/50 rounded-2xl shadow-sm transition-all hover:bg-slate-900/80" role="status">
+    <div className="flex justify-between items-center gap-2 p-3.5 bg-slate-900/50 border border-slate-800/50 rounded-2xl shadow-sm transition-all hover:bg-slate-900/80" role="status">
       <div className="flex items-center gap-2.5 min-w-0">
         {Icon && <Icon className="w-5 h-5 text-primary flex-shrink-0" aria-hidden="true" />}
         <span className="text-base md:text-lg text-slate-300 font-semibold truncate">