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/pointerevent.js

@@ -0,0 +1,88 @@
+var POINTER_INPUT_MAP = {
+    pointerdown: INPUT_START,
+    pointermove: INPUT_MOVE,
+    pointerup: INPUT_END,
+    pointercancel: INPUT_CANCEL,
+    pointerout: INPUT_CANCEL
+};
+
+// in IE10 the pointer types is defined as an enum
+var IE10_POINTER_TYPE_ENUM = {
+    2: INPUT_TYPE_TOUCH,
+    3: INPUT_TYPE_PEN,
+    4: INPUT_TYPE_MOUSE,
+    5: INPUT_TYPE_KINECT // see https://twitter.com/jacobrossi/status/480596438489890816
+};
+
+var POINTER_ELEMENT_EVENTS = 'pointerdown';
+var POINTER_WINDOW_EVENTS = 'pointermove pointerup pointercancel';
+
+// IE10 has prefixed support, and case-sensitive
+if (window.MSPointerEvent && !window.PointerEvent) {
+    POINTER_ELEMENT_EVENTS = 'MSPointerDown';
+    POINTER_WINDOW_EVENTS = 'MSPointerMove MSPointerUp MSPointerCancel';
+}
+
+/**
+ * Pointer events input
+ * @constructor
+ * @extends Input
+ */
+function PointerEventInput() {
+    this.evEl = POINTER_ELEMENT_EVENTS;
+    this.evWin = POINTER_WINDOW_EVENTS;
+
+    Input.apply(this, arguments);
+
+    this.store = (this.manager.session.pointerEvents = []);
+}
+
+inherit(PointerEventInput, Input, {
+    /**
+     * handle mouse events
+     * @param {Object} ev
+     */
+    handler: function PEhandler(ev) {
+        var store = this.store;
+        var removePointer = false;
+
+        var eventTypeNormalized = ev.type.toLowerCase().replace('ms', '');
+        var eventType = POINTER_INPUT_MAP[eventTypeNormalized];
+        var pointerType = IE10_POINTER_TYPE_ENUM[ev.pointerType] || ev.pointerType;
+
+        var isTouch = (pointerType == INPUT_TYPE_TOUCH);
+
+        // get index of the event in the store
+        var storeIndex = inArray(store, ev.pointerId, 'pointerId');
+
+        // start and mouse must be down
+        if (eventType & INPUT_START && (ev.button === 0 || isTouch)) {
+            if (storeIndex < 0) {
+                store.push(ev);
+                storeIndex = store.length - 1;
+            }
+        } else if (eventType & (INPUT_END | INPUT_CANCEL)) {
+            removePointer = true;
+        }
+
+        // it not found, so the pointer hasn't been down (so it's probably a hover)
+        if (storeIndex < 0) {
+            return;
+        }
+
+        // update the event in the store
+        store[storeIndex] = ev;
+
+        this.callback(this.manager, eventType, {
+            pointers: store,
+            changedPointers: [ev],
+            pointerType: pointerType,
+            srcEvent: ev
+        });
+
+        if (removePointer) {
+            // remove from the store
+            store.splice(storeIndex, 1);
+        }
+    }
+});