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/node-addon-api/common.gypi

@@ -0,0 +1,20 @@
+{
+  'variables': {
+    'NAPI_VERSION%': "<!(node -p \"process.env.NAPI_VERSION || process.versions.napi\")",
+    'disable_deprecated': "<!(node -p \"process.env['npm_config_disable_deprecated']\")"
+  },
+  'conditions': [
+    ['NAPI_VERSION!=""', { 'defines': ['NAPI_VERSION=<@(NAPI_VERSION)'] } ],
+    ['disable_deprecated=="true"', {
+      'defines': ['NODE_ADDON_API_DISABLE_DEPRECATED']
+    }],
+    ['OS=="mac"', {
+      'cflags+': ['-fvisibility=hidden'],
+      'xcode_settings': {
+        'OTHER_CFLAGS': ['-fvisibility=hidden']
+      }
+    }]
+  ],
+  'cflags': [ '-Werror', '-Wall', '-Wextra', '-Wpedantic', '-Wunused-parameter' ],
+  'cflags_cc': [ '-Werror', '-Wall', '-Wextra', '-Wpedantic', '-Wunused-parameter' ]
+}