feat: Add scroll-hide header and contextual FAB system

Implemented:

1. Scroll-Hide App Bar (Mobile)
   - Hides on scroll down (past 50px)
   - Shows on scroll up
   - Chrome URL bar behavior
   - Smooth slide animation (300ms)
   - Desktop always visible (md:translate-y-0)

2. Contextual FAB Hook
   - useFABConfig() hook for pages
   - Pre-configured for: orders, products, customers, coupons, dashboard
   - Automatic cleanup on unmount
   - Easy to use: useFABConfig('orders')

3. Removed Focus Styles
   - Bottom nav links: focus:outline-none
   - Cleaner mobile UX

Header Scroll Behavior:
- Scroll down > 50px: Header slides up (-translate-y-full)
- Scroll up: Header slides down (translate-y-0)
- Desktop: Always visible (md:translate-y-0)
- Smooth transition (duration-300)

FAB Configuration:
const configs = {
  orders: 'Create Order' → /orders/new
  products: 'Add Product' → /products/new
  customers: 'Add Customer' → /customers/new
  coupons: 'Create Coupon' → /coupons/new
  dashboard: 'Quick Actions' → (future speed dial)
  none: Hide FAB
}

Usage in Pages:
import { useFABConfig } from '@/hooks/useFABConfig';

function OrdersPage() {
  useFABConfig('orders'); // Sets up FAB automatically
  return <div>...</div>;
}

Next Steps:
- Add useFABConfig to actual pages
- Test scroll behavior on devices
- Implement speed dial for dashboard

Files Created:
- useFABConfig.tsx: Contextual FAB configuration hook

Files Modified:
- App.tsx: Scroll detection and header animation
- BottomNav.tsx: Removed focus outline styles

admin-spa/src/components/nav/BottomNav.tsx

@@ -63,7 +63,7 @@ export function BottomNav() {
             <NavLink
               key={item.to}
               to={item.to}
-              className={`flex flex-col items-center justify-center flex-1 h-full gap-0.5 transition-colors ${
+              className={`flex flex-col items-center justify-center flex-1 h-full gap-0.5 transition-colors focus:outline-none focus-visible:outline-none ${
                 active
                   ? 'text-primary'
                   : 'text-muted-foreground hover:text-foreground'