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/src/expose.js

@@ -0,0 +1,67 @@
+assign(Hammer, {
+    INPUT_START: INPUT_START,
+    INPUT_MOVE: INPUT_MOVE,
+    INPUT_END: INPUT_END,
+    INPUT_CANCEL: INPUT_CANCEL,
+
+    STATE_POSSIBLE: STATE_POSSIBLE,
+    STATE_BEGAN: STATE_BEGAN,
+    STATE_CHANGED: STATE_CHANGED,
+    STATE_ENDED: STATE_ENDED,
+    STATE_RECOGNIZED: STATE_RECOGNIZED,
+    STATE_CANCELLED: STATE_CANCELLED,
+    STATE_FAILED: STATE_FAILED,
+
+    DIRECTION_NONE: DIRECTION_NONE,
+    DIRECTION_LEFT: DIRECTION_LEFT,
+    DIRECTION_RIGHT: DIRECTION_RIGHT,
+    DIRECTION_UP: DIRECTION_UP,
+    DIRECTION_DOWN: DIRECTION_DOWN,
+    DIRECTION_HORIZONTAL: DIRECTION_HORIZONTAL,
+    DIRECTION_VERTICAL: DIRECTION_VERTICAL,
+    DIRECTION_ALL: DIRECTION_ALL,
+
+    Manager: Manager,
+    Input: Input,
+    TouchAction: TouchAction,
+
+    TouchInput: TouchInput,
+    MouseInput: MouseInput,
+    PointerEventInput: PointerEventInput,
+    TouchMouseInput: TouchMouseInput,
+    SingleTouchInput: SingleTouchInput,
+
+    Recognizer: Recognizer,
+    AttrRecognizer: AttrRecognizer,
+    Tap: TapRecognizer,
+    Pan: PanRecognizer,
+    Swipe: SwipeRecognizer,
+    Pinch: PinchRecognizer,
+    Rotate: RotateRecognizer,
+    Press: PressRecognizer,
+
+    on: addEventListeners,
+    off: removeEventListeners,
+    each: each,
+    merge: merge,
+    extend: extend,
+    assign: assign,
+    inherit: inherit,
+    bindFn: bindFn,
+    prefixed: prefixed
+});
+
+// this prevents errors when Hammer is loaded in the presence of an AMD
+//  style loader but by script tag, not by the loader.
+var freeGlobal = (typeof window !== 'undefined' ? window : (typeof self !== 'undefined' ? self : {})); // jshint ignore:line
+freeGlobal.Hammer = Hammer;
+
+if (typeof define === 'function' && define.amd) {
+    define(function() {
+        return Hammer;
+    });
+} else if (typeof module != 'undefined' && module.exports) {
+    module.exports = Hammer;
+} else {
+    window[exportName] = Hammer;
+}