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/tests/unit/gestures/test_pan.js

@@ -0,0 +1,63 @@
+var el,
+    hammer;
+
+module('Pan Gesture', {
+    setup: function() {
+        el = document.createElement('div');
+        document.body.appendChild(el);
+
+        hammer = new Hammer(el, {recognizers: []});
+    },
+    teardown: function() {
+        document.body.removeChild(el);
+        hammer.destroy();
+    }
+});
+
+test('`panstart` and `panmove` should be recognized', function() {
+    expect(2);
+
+    var panMoveCount = 0;
+    var pan = new Hammer.Pan({threshold: 1});
+
+    hammer.add(pan);
+    hammer.on('panstart', function() {
+      ok(true);
+    });
+    hammer.on('panmove', function() {
+      panMoveCount++;
+    });
+
+    utils.dispatchTouchEvent(el, 'start', 50, 50);
+    utils.dispatchTouchEvent(el, 'move', 70, 50);
+    utils.dispatchTouchEvent(el, 'move', 90, 50);
+
+    equal(panMoveCount, 1);
+});
+
+asyncTest('Pan event flow should be start -> left -> end', function() {
+    expect(1);
+    var pan = new Hammer.Pan({threshold: 1});
+    hammer.add(pan);
+
+    var eventflow = "";
+    var isCalledPanleft = false;
+    hammer.on('panstart', function() {
+        eventflow += "start";
+    });
+    hammer.on('panleft', function() {
+        if(!isCalledPanleft){
+            isCalledPanleft = true;
+            eventflow += "left";
+        }
+    });
+    hammer.on('panend', function() {
+        eventflow += "end";
+        isCalledPanleft = true;
+    });
+
+    Simulator.gestures.pan(el, { deltaX: -100, deltaY: 0 }, function() {
+        equal(eventflow,"startleftend");
+        start();
+    });
+});

LittleShop/wwwroot/lib/hammerjs/tests/unit/gestures/test_pinch.js

@@ -0,0 +1,43 @@
+var el,
+    hammer;
+
+module('Pinch Gesture', {
+    setup: function() {
+        el = document.createElement('div');
+        document.body.appendChild(el);
+
+        hammer = new Hammer(el, {recognizers: []});
+    },
+    teardown: function() {
+        document.body.removeChild(el);
+        hammer.destroy();
+    }
+});
+
+asyncTest('Pinch event flow should be start -> in -> end', function() {
+    expect(1);
+    var pinch = new Hammer.Pinch({enable: true, threshold: .1});
+    hammer.add(pinch);
+
+    var eventflow = "";
+    var isFiredPinchin = false;
+    hammer.on('pinchstart', function() {
+        eventflow += "start";
+    });
+    hammer.on('pinchin', function() {
+        if(!isFiredPinchin){
+            isFiredPinchin = true;
+            eventflow += "in";
+        }
+    });
+    hammer.on('pinchend', function() {
+        eventflow += "end";
+        isFiredPinchin = false;
+    });
+
+    Simulator.gestures.pinch(el, { duration: 500, scale: .5 }, function() {
+        equal(eventflow,"startinend");
+        start();
+    });
+});
+

LittleShop/wwwroot/lib/hammerjs/tests/unit/gestures/test_swipe.js

@@ -0,0 +1,29 @@
+var el,
+    hammer,
+    swipeCount = 0;
+
+module('Swipe Gesture', {
+    setup: function() {
+        el = utils.createHitArea();
+        hammer = new Hammer(el, {recognizers: []});
+        swipeCount = 0;
+    },
+    teardown: function() {
+        hammer.destroy();
+    }
+});
+
+test('swipe can be recognized', function() {
+    expect(1);
+
+    var swipe = new Hammer.Swipe({threshold: 1});
+    hammer.add(swipe);
+    hammer.on('swipe', function() {
+        ok(true);
+        start();
+    });
+
+    stop();
+
+    Simulator.gestures.swipe(el);
+});