feat: Phase 2.5 - Variant Collections Spreadsheet Editor

Replaces JSON textarea with professional Excel-like spreadsheet interface for managing product variant properties.

Features:
- Handsontable 14.6.1 spreadsheet component
- Property presets (Size, Color, Material, Storage, Custom)
- Inline cell editing with Tab/Enter navigation
- Context menu for add/remove rows and columns
- Keyboard shortcuts (Ctrl+D delete, Ctrl+Enter save, Ctrl+Z undo)
- Mobile touch gestures (swipe to delete rows)
- Automatic JSON serialization on form submit
- Form validation before saving
- Comprehensive user guide documentation

Files Changed:
- LittleShop/package.json: NPM package management setup
- LittleShop/wwwroot/js/variant-editor.js: 400-line spreadsheet editor module
- LittleShop/wwwroot/lib/handsontable/: Handsontable library (Community Edition)
- LittleShop/wwwroot/lib/hammerjs/: Hammer.js touch gesture library
- LittleShop/Areas/Admin/Views/VariantCollections/Edit.cshtml: Spreadsheet UI integration
- VARIANT_COLLECTIONS_USER_GUIDE.md: Complete user guide (18+ pages)

Technical Details:
- Excel-like editing experience (no more manual JSON editing)
- Mobile-first responsive design
- Browser compatibility: Chrome 90+, Firefox 88+, Edge 90+, Safari 14+
- Touch-optimized for mobile administration
- Automatic data validation and error handling

LittleShop/wwwroot/lib/hammerjs/tests/manual/assets/style.css

@@ -0,0 +1,42 @@
+@import url(http://fonts.googleapis.com/css?family=Open+Sans);
+
+*, *:after, *:before {
+    box-sizing: border-box;
+    -moz-box-sizing: border-box;
+}
+
+html, body {
+    margin: 0;
+    padding: 0;
+    height: 100%;
+    min-height: 100%;
+    background: #eee;
+    font: 13px/1.5em 'Open Sans', Helvetica, Arial, sans-serif;
+}
+
+a {
+    color: #4986e7;
+}
+
+.bg1, .green { background: #42d692; }
+.bg2, .blue { background: #4986e7; }
+.bg3, .red { background: #d06b64; }
+.bg4, .purple { background: #cd74e6; }
+.bg5, .azure { background: #9fe1e7; }
+
+body {
+    margin: 20px;
+}
+
+pre {
+    background: #fff;
+    padding: 20px;
+    margin-bottom: 20px;
+}
+
+.container {
+    max-width: 900px;
+    margin: 0 auto;
+}
+
+.clear { clear: both; }