feat: Implement mobile-first navigation with bottom bar and FAB

Implemented mobile-optimized navigation structure:

1. Bottom Navigation (Mobile Only)
   - 5 items: Dashboard, Orders, Products, Customers, More
   - Fixed at bottom, always visible
   - Thumb-friendly positioning
   - Active state indication
   - Hidden on desktop (md:hidden)

2. More Menu Page
   - Overflow menu for Coupons and Settings
   - Clean list layout with icons
   - Descriptions for each item
   - Chevron indicators

3. FAB (Floating Action Button)
   - Context-aware system via FABContext
   - Fixed bottom-right (72px from bottom)
   - Hidden on desktop (md:hidden)
   - Ready for contextual actions per page

4. FAB Context System
   - Global state for FAB configuration
   - setFAB() / clearFAB() methods
   - Supports icon, label, onClick, visibility
   - Allows pages to control FAB behavior

5. Layout Updates
   - Added pb-14 to main for bottom nav spacing
   - BottomNav and FAB in mobile fullscreen layout
   - Wrapped app with FABProvider

Structure (Mobile):
┌─────────────────────────────────┐
│ App Bar (will hide on scroll)   │
├─────────────────────────────────┤
│ Page Header (sticky, contextual)│
├─────────────────────────────────┤
│ Submenu (sticky)                │
├─────────────────────────────────┤
│ Content (scrollable)            │
│                        [+] FAB  │
├─────────────────────────────────┤
│ Bottom Nav (fixed)              │
└─────────────────────────────────┘

Next Steps:
- Implement scroll-hide for app bar
- Add contextual FAB per page
- Test on real devices

Files Created:
- BottomNav.tsx: Bottom navigation component
- More/index.tsx: More menu page
- FABContext.tsx: FAB state management
- FAB.tsx: Floating action button component
- useScrollDirection.ts: Scroll detection hook

Files Modified:
- App.tsx: Added bottom nav, FAB, More route, providers

admin-spa/src/components/FAB.tsx

@@ -0,0 +1,23 @@
+import React from 'react';
+import { Plus } from 'lucide-react';
+import { useFAB } from '@/contexts/FABContext';
+import { Button } from '@/components/ui/button';
+
+export function FAB() {
+  const { config } = useFAB();
+
+  if (!config || config.visible === false) {
+    return null;
+  }
+
+  return (
+    <Button
+      onClick={config.onClick}
+      size="lg"
+      className="fixed bottom-[72px] right-4 z-40 w-14 h-14 rounded-2xl shadow-lg hover:shadow-xl transition-all duration-200 md:hidden"
+      aria-label={config.label}
+    >
+      {config.icon || <Plus className="w-6 h-6" />}
+    </Button>
+  );
+}