feat: Phase 1 - Critical WCAG 2.1 AA accessibility improvements

Implemented comprehensive accessibility enhancements to meet WCAG 2.1 AA standards:

**Skip Navigation:**
- Added skip-to-content link for keyboard users
- Link appears on focus and jumps directly to main content area

**Screen Reader Support:**
- Created .sr-only and .sr-only-focusable utility classes
- Added aria-hidden="true" to all decorative icons
- Added descriptive aria-label attributes to all icon-only buttons

**Enhanced Focus Indicators:**
- Implemented 3px visible outlines on all interactive elements
- Added :focus-visible for keyboard-only focus indicators
- Special focus styling for primary actions (orange outline)
- Consistent 2px outline-offset for better visibility

**Table Accessibility:**
- Added scope="col" attributes to all table headers
- Properly grouped button actions with role="group" and aria-label

**Button Improvements:**
- All icon-only buttons now have descriptive ARIA labels
- Added responsive text labels (visible on sm+ screens, hidden on mobile)
- Improved button groups with proper ARIA roles

**Files Modified:**
- _Layout.cshtml: Skip link, accessible menu close button
- Categories/Index.cshtml: ARIA labels, table scopes
- Users/Index.cshtml: ARIA labels, table scopes
- Orders/Index.cshtml: Table scopes
- Products/Index.cshtml: Table scopes
- ShippingRates/Index.cshtml: ARIA labels, table scopes
- VariantCollections/Index.cshtml: ARIA labels, table scopes
- modern-admin.css: Accessibility utilities and enhanced focus styles

**WCAG 2.1 AA Criteria Addressed:**
- 2.4.1 Bypass Blocks (Level A)
- 2.4.7 Focus Visible (Level AA)
- 4.1.2 Name, Role, Value (Level A)
- 1.3.1 Info and Relationships (Level A)

🚀 Generated with Claude Code

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

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

@@ -108,13 +108,13 @@
                 <table class="table table-hover">
                     <thead>
                         <tr>
-                            <th>Order ID</th>
-                            <th>Customer</th>
-                            <th>Items</th>
-                            <th>Total</th>
-                            <th>Status</th>
-                            <th>Timeline</th>
-                            <th>Actions</th>
+                            <th scope="col">Order ID</th>
+                            <th scope="col">Customer</th>
+                            <th scope="col">Items</th>
+                            <th scope="col">Total</th>
+                            <th scope="col">Status</th>
+                            <th scope="col">Timeline</th>
+                            <th scope="col">Actions</th>
                         </tr>
                     </thead>
                     <tbody>