feat: Phase 2 - Mobile-first responsive card views

Implemented responsive mobile card layouts for all main Index views, providing superior mobile UX while maintaining desktop table views.

**Responsive Design Pattern:**
- Desktop (≥992px): Table layout with all data columns
- Mobile (<992px): Card-based layout optimized for touch interaction
- Breakpoint: Bootstrap's lg breakpoint for optimal viewing experience

**Views Converted:**

1. **Categories/Index.cshtml:**
   - Mobile cards with name, description, product count, status
   - Full-width action buttons for easy touch interaction
   - Clear visual hierarchy with icons and badges

2. **Users/Index.cshtml:**
   - Simplified mobile cards showing username, created date, status
   - Conditional delete button (protected admin account)
   - Clean, minimal design for quick user management

3. **ShippingRates/Index.cshtml:**
   - 2x2 grid layout for shipping rate data (country, price, weight, delivery)
   - Visual separation with light background boxes
   - All critical information displayed in scannable format

4. **VariantCollections/Index.cshtml:**
   - Properties JSON displayed in scrollable code block
   - Created/Updated dates in compact format
   - Clear deactivation action for variant collections

**Mobile UX Enhancements:**
- ✅ 44px minimum touch targets (Bootstrap .btn default)
- ✅ Full-width buttons with .d-grid gap-2 for easy tapping
- ✅ Proper spacing with mb-3 between cards
- ✅ Clear visual hierarchy with card-title and badges
- ✅ Descriptive button text (not just icons) on mobile
- ✅ Responsive icons and status indicators
- ✅ Word-break handling for long JSON strings

**Technical Implementation:**
- Used Bootstrap's d-none d-lg-block for desktop tables
- Used d-lg-none for mobile card views
- No JavaScript required - pure CSS responsive design
- Maintains all functionality from desktop view
- Zero data loss in mobile transformation

**Accessibility Maintained:**
- All ARIA labels preserved from Phase 1
- Semantic HTML structure in both views
- Proper heading hierarchy maintained
- Keyboard navigation fully functional

🚀 Generated with Claude Code

Co-Authored-By: Claude <noreply@anthropic.com>

LittleShop/Areas/Admin/Views/Users/Index.cshtml

@@ -35,7 +35,8 @@
     <div class="card-body">
         @if (Model.Any())
         {
-            <div class="table-responsive">
+            <!-- Desktop Table View (hidden on mobile) -->
+            <div class="table-responsive d-none d-lg-block">
                 <table class="table table-striped">
                     <thead>
                         <tr>
@@ -83,6 +84,50 @@
                     </tbody>
                 </table>
             </div>
+
+            <!-- Mobile Card View (visible on mobile only) -->
+            <div class="d-lg-none">
+                @foreach (var user in Model)
+                {
+                    <div class="card mb-3">
+                        <div class="card-body">
+                            <div class="d-flex justify-content-between align-items-start mb-2">
+                                <h5 class="card-title mb-0">
+                                    <i class="fas fa-user text-primary"></i> @user.Username
+                                </h5>
+                                @if (user.IsActive)
+                                {
+                                    <span class="badge bg-success">Active</span>
+                                }
+                                else
+                                {
+                                    <span class="badge bg-danger">Inactive</span>
+                                }
+                            </div>
+
+                            <p class="text-muted mb-3">
+                                <i class="fas fa-calendar" aria-hidden="true"></i> Created: @user.CreatedAt.ToString("MMM dd, yyyy")
+                            </p>
+
+                            <div class="d-grid gap-2">
+                                <a href="@Url.Action("Edit", new { id = user.Id })" class="btn btn-outline-primary" aria-label="Edit @user.Username">
+                                    <i class="fas fa-edit" aria-hidden="true"></i> Edit User
+                                </a>
+                                @if (user.Username != "admin")
+                                {
+                                    <form method="post" action="@Url.Action("Delete", new { id = user.Id })"
+                                          onsubmit="return confirm('Are you sure you want to delete this user?')">
+                                        @Html.AntiForgeryToken()
+                                        <button type="submit" class="btn btn-outline-danger w-100" aria-label="Delete @user.Username">
+                                            <i class="fas fa-trash" aria-hidden="true"></i> Delete User
+                                        </button>
+                                    </form>
+                                }
+                            </div>
+                        </div>
+                    </div>
+                }
+            </div>
         }
         else
         {