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/assets/qunit.css

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+/*!
+ * QUnit 1.14.0
+ * http://qunitjs.com/
+ *
+ * Copyright 2013 jQuery Foundation and other contributors
+ * Released under the MIT license
+ * http://jquery.org/license
+ *
+ * Date: 2014-01-31T16:40Z
+ */
+
+/** Font Family and Sizes */
+
+#qunit-tests, #qunit-header, #qunit-banner, #qunit-testrunner-toolbar, #qunit-userAgent, #qunit-testresult {
+	font-family: "Helvetica Neue Light", "HelveticaNeue-Light", "Helvetica Neue", Calibri, Helvetica, Arial, sans-serif;
+}
+
+#qunit-testrunner-toolbar, #qunit-userAgent, #qunit-testresult, #qunit-tests li { font-size: small; }
+#qunit-tests { font-size: smaller; }
+
+
+/** Resets */
+
+#qunit-tests, #qunit-header, #qunit-banner, #qunit-userAgent, #qunit-testresult, #qunit-modulefilter {
+	margin: 0;
+	padding: 0;
+}
+
+
+/** Header */
+
+#qunit-header {
+	padding: 0.5em 0 0.5em 1em;
+
+	color: #8699A4;
+	background-color: #0D3349;
+
+	font-size: 1.5em;
+	line-height: 1em;
+	font-weight: 400;
+
+	border-radius: 5px 5px 0 0;
+}
+
+#qunit-header a {
+	text-decoration: none;
+	color: #C2CCD1;
+}
+
+#qunit-header a:hover,
+#qunit-header a:focus {
+	color: #FFF;
+}
+
+#qunit-testrunner-toolbar label {
+	display: inline-block;
+	padding: 0 0.5em 0 0.1em;
+}
+
+#qunit-banner {
+	height: 5px;
+}
+
+#qunit-testrunner-toolbar {
+	padding: 0.5em 0 0.5em 2em;
+	color: #5E740B;
+	background-color: #EEE;
+	overflow: hidden;
+}
+
+#qunit-userAgent {
+	padding: 0.5em 0 0.5em 2.5em;
+	background-color: #2B81AF;
+	color: #FFF;
+	text-shadow: rgba(0, 0, 0, 0.5) 2px 2px 1px;
+}
+
+#qunit-modulefilter-container {
+	float: right;
+}
+
+/** Tests: Pass/Fail */
+
+#qunit-tests {
+	list-style-position: inside;
+}
+
+#qunit-tests li {
+	padding: 0.4em 0.5em 0.4em 2.5em;
+	border-bottom: 1px solid #FFF;
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+}
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+}
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+#qunit-tests ins {
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+	color: #500;
+	text-decoration: none;
+}
+
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+
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+}
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+/*** Passing Styles */
+
+#qunit-tests li li.pass {
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+}
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+
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+#qunit-tests .fail .test-expected           { color: #008000; }
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+#qunit-banner.qunit-fail                    { background-color: #EE5757; }
+
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+/** Result */
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+	padding: 0.5em 0.5em 0.5em 2.5em;
+
+	color: #2B81AF;
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+}
+#qunit-testresult .module-name {
+	font-weight: 700;
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+}

LittleShop/wwwroot/lib/hammerjs/tests/unit/assets/utils.js

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+var utils = {
+    /**
+     * trigger simple dom event
+     * @param obj
+     * @param name
+     */
+    triggerDomEvent: function(obj, name) {
+        var event = document.createEvent('Event');
+        event.initEvent(name, true, true);
+        obj.dispatchEvent(event);
+    },
+
+
+    createTouchEvent: function(name, x, y, identifier) {
+        var event = document.createEvent('Event');
+        event.initEvent('touch' + name, true, true);
+
+        event.touches = event.targetTouches = [{
+            clientX: x,
+            clientY: y,
+            identifier: identifier || 0
+        }];
+
+        //https://developer.mozilla.org/en-US/docs/Web/API/TouchEvent.changedTouches
+        event.changedTouches = [{
+            clientX: x,
+            clientY: y,
+            identifier: identifier || 0
+        }];
+
+        return event;
+    },
+
+    dispatchTouchEvent: function(el, name, x, y, identifier) {
+        var event = utils.createTouchEvent(name, x, y, identifier);
+        el.dispatchEvent(event);
+    },
+
+    createHitArea: function(parent) {
+        if (parent == null) {
+            parent = document.getElementById('qunit-fixture')
+        }
+        var hitArea = document.createElement('div');
+        hitArea.style.background = '#eee';
+        hitArea.style.height = '300px';
+
+        parent.appendChild(hitArea);
+        return hitArea;
+    }
+};