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/LICENSE.md

@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (C) 2011-2014 by Jorik Tangelder (Eight Media)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.