fix: Implement responsive Drawer for payment gateway settings on mobile

Problem: Payment gateway settings modal was using Dialog on all screen sizes
Solution: Split into responsive Dialog (desktop) and Drawer (mobile)

Changes:
1. Added Drawer and useMediaQuery imports
2. Added isDesktop hook: useMediaQuery("(min-width: 768px)")
3. Split modal into two conditional renders:
   - Desktop (≥768px): Dialog with horizontal footer layout
   - Mobile (<768px): Drawer with vertical footer layout

Desktop Layout (Dialog):
- Center modal overlay
- Horizontal footer: Cancel | View in WC | Save
- max-h-[80vh] for scrolling

Mobile Layout (Drawer):
- Bottom sheet (slides up from bottom)
- Vertical footer (full width buttons):
  1. Save Settings (primary)
  2. View in WooCommerce (ghost)
  3. Cancel (outline)
- max-h-[90vh] for more screen space
- Swipe down to dismiss

Benefits:
✅ Native mobile experience with bottom sheet
✅ Easier to reach buttons on mobile (bottom of screen)
✅ Better one-handed use
✅ Swipe gesture to dismiss
✅ Desktop keeps familiar modal experience

User Changes Applied:
- AlertDialog z-index: z-50 → z-[999] (higher than other modals)
- Dialog max-height: max-h-[100vh] → max-h-[80vh] (better desktop UX)

Files Modified:
- Payments.tsx: Responsive Dialog/Drawer implementation
- alert-dialog.tsx: Increased z-index for proper layering

admin-spa/src/App.tsx

@@ -57,7 +57,7 @@ function useFullscreen() {
         .wnw-fullscreen .woonoow-fullscreen-root {
           position: fixed;
           inset: 0;
-          z-index: 9999;
+          z-index: 999;
           background: var(--background, #fff);
           height: 100dvh; /* ensure full viewport height on mobile/desktop */
           overflow: hidden; /* prevent double scrollbars; inner <main> handles scrolling */