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/Gruntfile.coffee

@@ -0,0 +1,124 @@
+module.exports = (grunt) ->
+  grunt.initConfig
+    pkg: grunt.file.readJSON 'package.json'
+
+    usebanner:
+      taskName:
+        options:
+          position: 'top'
+          banner: '
+/*! <%= pkg.title || pkg.name %> - v<%= pkg.version %> - <%= grunt.template.today("yyyy-mm-dd") %>\n
+ * <%= pkg.homepage %>\n
+ *\n
+ * Copyright (c) <%= grunt.template.today("yyyy") %> <%= pkg.author.name %>;\n
+ * Licensed under the <%= pkg.license %> license */'
+          linebreak: true
+        files:
+          src: ['./hammer.js','./hammer.min.js']
+
+    concat:
+      build:
+        src: [
+          'src/hammer.prefix.js'
+          'src/utils.js'
+          'src/input.js'
+          'src/input/*.js'
+          'src/touchaction.js'
+          'src/recognizer.js'
+          'src/recognizers/*.js'
+          'src/hammer.js'
+          'src/manager.js'
+          'src/expose.js'
+          'src/hammer.suffix.js']
+        dest: 'hammer.js'
+
+    uglify:
+      min:
+        options:
+          report: 'gzip'
+          sourceMap: 'hammer.min.map'
+        files:
+          'hammer.min.js': ['hammer.js']
+       # special test build that exposes everything so it's testable
+      test:
+        options:
+          wrap: "$H"
+          comments: 'all'
+          exportAll: true
+          mangle: false
+          beautify: true
+          compress:
+            global_defs:
+              exportName: 'Hammer'
+        files:
+          'tests/build.js': [
+            'src/utils.js'
+            'src/input.js'
+            'src/input/*.js'
+            'src/touchaction.js'
+            'src/recognizer.js'
+            'src/recognizers/*.js'
+            'src/hammer.js'
+            'src/manager.js'
+            'src/expose.js']
+
+    'string-replace':
+      version:
+        files:
+          'hammer.js': 'hammer.js'
+        options:
+          replacements: [
+              pattern: '{{PKG_VERSION}}'
+              replacement: '<%= pkg.version %>'
+            ]
+
+    jshint:
+      options:
+        jshintrc: true
+      build:
+        src: ['hammer.js']
+
+    jscs:
+      src: [
+        'src/**/*.js',
+        '!src/hammer.prefix.js',
+        '!src/hammer.suffix.js'
+      ]
+      options:
+        config: "./.jscsrc"
+        force: true
+
+    watch:
+      scripts:
+        files: ['src/**/*.js']
+        tasks: ['concat','string-replace','uglify','jshint','jscs']
+        options:
+          interrupt: true
+
+    connect:
+      server:
+        options:
+          hostname: "0.0.0.0"
+          port: 8000
+
+    qunit:
+      all: ['tests/unit/index.html']
+
+
+  # Load tasks
+  grunt.loadNpmTasks 'grunt-contrib-concat'
+  grunt.loadNpmTasks 'grunt-contrib-uglify'
+  grunt.loadNpmTasks 'grunt-contrib-qunit'
+  grunt.loadNpmTasks 'grunt-contrib-watch'
+  grunt.loadNpmTasks 'grunt-contrib-jshint'
+  grunt.loadNpmTasks 'grunt-contrib-connect'
+  grunt.loadNpmTasks 'grunt-string-replace'
+  grunt.loadNpmTasks 'grunt-banner'
+  grunt.loadNpmTasks 'grunt-jscs'
+
+  # Default task(s)
+  grunt.registerTask 'default', ['connect', 'watch']
+  grunt.registerTask 'default-test', ['connect', 'uglify:test', 'watch']
+  grunt.registerTask 'build', ['concat', 'string-replace', 'uglify:min', 'usebanner', 'test']
+  grunt.registerTask 'test', ['jshint', 'jscs', 'uglify:test', 'qunit']
+  grunt.registerTask 'test-travis', ['build']