fix(mobile): tab bars scroll on phones + combobox 44px touch targets

- .admin-tabs: horizontal-scroll + 44px tabs at <=640px so 4-tab bars
  (WarehouseReports, Invoices/Offers status filters, OffersTemplates) no longer
  overflow off-screen.
- Customer searchable dropdown + warehouse ItemPicker: option rows >=44px,
  clear button enlarged, list clamped to viewport width at <=640px.
Native selects/toggles were already mobile-safe (44px at <=768px). Mobile-only; desktop unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/forms.css

@@ -488,3 +488,23 @@
   color: var(--text-tertiary);
   font-size: 0.8125rem;
 }
+
+@media (max-width: 640px) {
+  .admin-customer-dropdown {
+    max-width: calc(100vw - 24px);
+    overflow-y: auto;
+  }
+
+  .admin-customer-dropdown-item {
+    display: flex;
+    flex-direction: column;
+    justify-content: center;
+    min-height: 44px;
+  }
+
+  .admin-customer-selected .admin-btn-icon {
+    min-width: 44px;
+    min-height: 44px;
+    margin-right: -10px;
+  }
+}