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/input/singletouch.js

@@ -0,0 +1,68 @@
+var SINGLE_TOUCH_INPUT_MAP = {
+    touchstart: INPUT_START,
+    touchmove: INPUT_MOVE,
+    touchend: INPUT_END,
+    touchcancel: INPUT_CANCEL
+};
+
+var SINGLE_TOUCH_TARGET_EVENTS = 'touchstart';
+var SINGLE_TOUCH_WINDOW_EVENTS = 'touchstart touchmove touchend touchcancel';
+
+/**
+ * Touch events input
+ * @constructor
+ * @extends Input
+ */
+function SingleTouchInput() {
+    this.evTarget = SINGLE_TOUCH_TARGET_EVENTS;
+    this.evWin = SINGLE_TOUCH_WINDOW_EVENTS;
+    this.started = false;
+
+    Input.apply(this, arguments);
+}
+
+inherit(SingleTouchInput, Input, {
+    handler: function TEhandler(ev) {
+        var type = SINGLE_TOUCH_INPUT_MAP[ev.type];
+
+        // should we handle the touch events?
+        if (type === INPUT_START) {
+            this.started = true;
+        }
+
+        if (!this.started) {
+            return;
+        }
+
+        var touches = normalizeSingleTouches.call(this, ev, type);
+
+        // when done, reset the started state
+        if (type & (INPUT_END | INPUT_CANCEL) && touches[0].length - touches[1].length === 0) {
+            this.started = false;
+        }
+
+        this.callback(this.manager, type, {
+            pointers: touches[0],
+            changedPointers: touches[1],
+            pointerType: INPUT_TYPE_TOUCH,
+            srcEvent: ev
+        });
+    }
+});
+
+/**
+ * @this {TouchInput}
+ * @param {Object} ev
+ * @param {Number} type flag
+ * @returns {undefined|Array} [all, changed]
+ */
+function normalizeSingleTouches(ev, type) {
+    var all = toArray(ev.touches);
+    var changed = toArray(ev.changedTouches);
+
+    if (type & (INPUT_END | INPUT_CANCEL)) {
+        all = uniqueArray(all.concat(changed), 'identifier', true);
+    }
+
+    return [all, changed];
+}