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/BottomNav.tsx

@@ -28,27 +28,27 @@ export default function BottomNav({
     <footer className="fixed bottom-0 left-0 right-0 py-3 px-2 pb-safe bg-slate-950/80 backdrop-blur-lg border-t border-slate-900/50 z-40 transition-all duration-300">
       <div className="max-w-xl mx-auto flex justify-around items-center text-slate-400">
         
-        <button 
+        <button
           onClick={() => router.push('/')}
-          className={cn("flex flex-col items-center gap-1", isHome && "text-primary")}
+          className={cn("flex flex-col items-center gap-1 rounded px-2 py-1 focus-visible:ring-2 focus-visible:ring-primary focus-visible:outline-none", isHome && "text-primary")}
         >
           <Smartphone size={20} />
           <span className="text-xs font-bold transition-all">Home</span>
         </button>
 
         {/* Inventory */}
-        <button 
+        <button
           onClick={() => router.push('/inventory')}
-          className={cn("flex flex-col items-center gap-1", isInventory && "text-primary")}
+          className={cn("flex flex-col items-center gap-1 rounded px-2 py-1 focus-visible:ring-2 focus-visible:ring-primary focus-visible:outline-none", isInventory && "text-primary")}
         >
           <Package size={20} />
           <span className="text-xs font-bold transition-all">Inventory</span>
         </button>
 
         {/* Logs */}
-        <button 
+        <button
           onClick={() => router.push('/logs')}
-          className={cn("flex flex-col items-center gap-1", isLogs && "text-primary")}
+          className={cn("flex flex-col items-center gap-1 rounded px-2 py-1 focus-visible:ring-2 focus-visible:ring-primary focus-visible:outline-none", isLogs && "text-primary")}
         >
           <History size={20} />
           <span className="text-xs font-bold transition-all">Logs</span>
@@ -56,9 +56,9 @@ export default function BottomNav({
 
         {/* Admin Settings */}
         {currentUser?.role === 'admin' && (
-          <button 
+          <button
             onClick={() => router.push('/admin')}
-            className={cn("flex flex-col items-center gap-1", isAdmin && "text-primary")}
+            className={cn("flex flex-col items-center gap-1 rounded px-2 py-1 focus-visible:ring-2 focus-visible:ring-primary focus-visible:outline-none", isAdmin && "text-primary")}
           >
             <Settings size={20} />
             <span className="text-xs font-bold transition-all">Admin</span>
@@ -66,14 +66,14 @@ export default function BottomNav({
         )}
 
         {/* Logout */}
-        <button 
+        <button
           onClick={() => {
             if (window.confirm("Are you sure you want to logout?")) {
               import('@/lib/auth').then(m => m.clearAuth());
               window.location.href = '/login';
             }
           }}
-          className="flex flex-col items-center gap-1 text-rose-500 hover:text-rose-400 transition-colors"
+          className="flex flex-col items-center gap-1 text-rose-500 hover:text-rose-400 transition-colors rounded px-2 py-1 focus-visible:ring-2 focus-visible:ring-rose-500 focus-visible:outline-none"
         >
           <LogOut size={20} />
           <span className="text-xs font-bold transition-all">Logout</span>