feat(shopinbit): step navigation and category selection UX

diff --git a/lib/pages/shopinbit/shopinbit_step_1.dart b/lib/pages/shopinbit/shopinbit_step_1.dart
index c7b35e4..5bdb7a0 100644
--- a/lib/pages/shopinbit/shopinbit_step_1.dart
+++ b/lib/pages/shopinbit/shopinbit_step_1.dart
@@ -12,7 +12,7 @@ import '../../widgets/desktop/desktop_dialog_close_button.dart';
 import '../../widgets/desktop/primary_button.dart';
 import '../../widgets/stack_text_field.dart';
 import '../exchange_view/sub_widgets/step_row.dart';
-import 'shopinbit_step_3.dart';
+import 'shopinbit_step_2.dart';
 
 class ShopInBitStep1 extends StatefulWidget {
   const ShopInBitStep1({super.key, required this.model});
@@ -51,18 +51,16 @@ class _ShopInBitStep1State extends State<ShopInBitStep1> {
 
   void _continue() {
     widget.model.displayName = _nameController.text.trim();
-    // Skip step 2 (category selection): only concierge is available initially
-    widget.model.category = ShopInBitCategory.concierge;
     if (Util.isDesktop) {
       Navigator.of(context, rootNavigator: true).pop();
       showDialog<void>(
         context: context,
-        builder: (_) => ShopInBitStep3(model: widget.model),
+        builder: (_) => ShopInBitStep2(model: widget.model),
       );
     } else {
       Navigator.of(
         context,
-      ).pushNamed(ShopInBitStep3.routeName, arguments: widget.model);
+      ).pushNamed(ShopInBitStep2.routeName, arguments: widget.model);
     }
   }
 
diff --git a/lib/pages/shopinbit/shopinbit_step_2.dart b/lib/pages/shopinbit/shopinbit_step_2.dart
index c7aaf82..31abd1d 100644
--- a/lib/pages/shopinbit/shopinbit_step_2.dart
+++ b/lib/pages/shopinbit/shopinbit_step_2.dart
@@ -31,7 +31,9 @@ class _ShopInBitStep2State extends State<ShopInBitStep2> {
   @override
   void initState() {
     super.initState();
-    _selected = widget.model.category;
+    // Reset category selection.
+    widget.model.category = null;
+    _selected = null;
   }
 
   void _continue() {
@@ -73,10 +75,25 @@ class _ShopInBitStep2State extends State<ShopInBitStep2> {
         padding: EdgeInsets.all(isDesktop ? 20 : 16),
         child: Row(
           children: [
-            SvgPicture.asset(
-              iconAsset,
-              width: isDesktop ? 32 : 24,
-              height: isDesktop ? 32 : 24,
+            Container(
+              width: isDesktop ? 48 : 40,
+              height: isDesktop ? 48 : 40,
+              decoration: BoxDecoration(
+                shape: BoxShape.circle,
+                color: Theme.of(context)
+                    .extension<StackColors>()!
+                    .accentColorBlue
+                    .withOpacity(0.1),
+              ),
+              alignment: Alignment.center,
+              child: SvgPicture.asset(
+                iconAsset,
+                width: isDesktop ? 24 : 20,
+                height: isDesktop ? 24 : 20,
+                color: Theme.of(context)
+                    .extension<StackColors>()!
+                    .accentColorBlue,
+              ),
             ),
             SizedBox(width: isDesktop ? 16 : 12),
             Expanded(
diff --git a/lib/pages/shopinbit/shopinbit_step_4.dart b/lib/pages/shopinbit/shopinbit_step_4.dart
index 44e17d2..05577b8 100644
--- a/lib/pages/shopinbit/shopinbit_step_4.dart
+++ b/lib/pages/shopinbit/shopinbit_step_4.dart
@@ -214,11 +214,16 @@ class _ShopInBitStep4State extends State<ShopInBitStep4> {
       final service = ShopInBitService.instance;
       final customerKey = await service.ensureCustomerKey();
 
+      assert(
+        widget.model.category != null,
+        'Step 4 reached with null category — Step 2 must set category before reaching Step 4',
+      );
+
       final categoryStr = switch (widget.model.category) {
         ShopInBitCategory.concierge => "concierge",
         ShopInBitCategory.travel => "travel",
         ShopInBitCategory.car => "car",
-        null => "concierge",
+        null => throw StateError('category must be non-null at Step 4 submit'),
       };
 
       final resp = await service.client.createRequest(
@@ -283,6 +288,11 @@ class _ShopInBitStep4State extends State<ShopInBitStep4> {
   Widget build(BuildContext context) {
     final isDesktop = Util.isDesktop;
 
+    final String descriptionPlaceholder =
+        widget.model.category == ShopInBitCategory.car
+            ? "Describe the car (make, model, year, requirements)"
+            : "What would you like to purchase?";
+
     final content = Column(
       crossAxisAlignment: CrossAxisAlignment.stretch,
       children: [
@@ -329,7 +339,7 @@ class _ShopInBitStep4State extends State<ShopInBitStep4> {
                 : STextStyles.field(context),
             decoration:
                 standardInputDecoration(
-                  "What would you like to purchase?",
+                  descriptionPlaceholder,
                   _descriptionFocusNode,
                   context,
                   desktopMed: isDesktop,