fix: Vertical tabs visibility and add mobile search/filter

Fixed 3 critical issues:

1. Fixed Vertical Tabs - Cards All Showing
   - Updated VerticalTabForm to hide inactive sections
   - Only active section visible (className: hidden for others)
   - Proper tab switching now works

2. Added Mobile Search/Filter to Coupons
   - Created CouponFilterSheet component
   - Added mobile search bar with icon
   - Filter button with active count badge
   - Matches Products pattern exactly
   - Sheet with Apply/Reset buttons

3. Removed max-height from VerticalTabForm
   - User removed max-h-[calc(100vh-200px)]
   - Content now flows naturally
   - Better for forms with varying heights

Components Created:
- CouponFilterSheet.tsx - Mobile filter bottom sheet
  - Discount type filter
  - Apply/Reset actions
  - Active filter count

Changes to Coupons/index.tsx:
- Added mobile search bar (md:hidden)
- Added filter sheet state
- Added activeFiltersCount
- Search icon + SlidersHorizontal icon
- Filter badge indicator

Changes to VerticalTabForm:
- Hide inactive sections (className: hidden)
- Only show section matching activeTab
- Proper visibility control

Result:
✅ Vertical tabs work correctly (only one section visible)
✅ Mobile search/filter on Coupons (like Products)
✅ Filter count badge
✅ Professional mobile UX

Next: Move customer site member checkbox to settings

admin-spa/src/routes/Coupons/New.tsx

@@ -53,7 +53,7 @@ export default function CouponNew() {
   }, [createMutation.isPending, setPageHeader, clearPageHeader, navigate]);
 
   return (
-    <div className="max-w-4xl">
+    <div>
       <CouponForm
         mode="create"
         onSubmit={async (data) => {