fix: prevent asset conflicts between React and Grid.js versions

Add coexistence checks to all enqueue methods to prevent loading
both React and Grid.js assets simultaneously.

Changes:
- ReactAdmin.php: Only enqueue React assets when ?react=1
- Init.php: Skip Grid.js when React active on admin pages
- Form.php, Coupon.php, Access.php: Restore classic assets when ?react=0
- Customer.php, Product.php, License.php: Add coexistence checks

Now the toggle between Classic and React versions works correctly.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

node_modules/@wordpress/base-styles/_long-content-fade.scss

@@ -0,0 +1,61 @@
+/**
+ * Long content fade mixin
+ *
+ * Creates a fading overlay to signify that the content is longer
+ * than the space allows.
+ */
+
+@mixin long-content-fade($direction: right, $size: 20%, $color: #fff, $edge: 0, $z-index: false) {
+	content: "";
+	display: block;
+	position: absolute;
+	-webkit-touch-callout: none;
+	-webkit-user-select: none;
+	-khtml-user-select: none;
+	-moz-user-select: none;
+	-ms-user-select: none;
+	user-select: none;
+	pointer-events: none;
+
+	@if $z-index {
+		z-index: $z-index;
+	}
+
+	@if $direction == "bottom" {
+		background: linear-gradient(to top, transparent, $color 90%);
+		left: $edge;
+		right: $edge;
+		top: $edge;
+		bottom: calc(100% - $size);
+		width: auto;
+	}
+
+	@if $direction == "top" {
+		background: linear-gradient(to bottom, transparent, $color 90%);
+		top: calc(100% - $size);
+		left: $edge;
+		right: $edge;
+		bottom: $edge;
+		width: auto;
+	}
+
+	@if $direction == "left" {
+		background: linear-gradient(to left, transparent, $color 90%);
+		top: $edge;
+		left: $edge;
+		bottom: $edge;
+		right: auto;
+		width: $size;
+		height: auto;
+	}
+
+	@if $direction == "right" {
+		background: linear-gradient(to right, transparent, $color 90%);
+		top: $edge;
+		bottom: $edge;
+		right: $edge;
+		left: auto;
+		width: $size;
+		height: auto;
+	}
+}