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/third-party-web/lib/markdown/updates/2019-05-13.md

@@ -0,0 +1,14 @@
+A shortcoming of the attribution approach has been fixed. Total usage is now reported based on the number of _pages_ in the dataset that use the third-party, not the number of _scripts_. Correspondingly, all average impact times are now reported _per page_ rather than _per script_. Previously, a third party could appear to have a lower impact or be more popular simply by splitting their work across multiple files.
+
+Third-parties that performed most of their work from a single script should see little to no impact from this change, but some entities have seen significant ranking movement. Hosting providers that host entire pages are, understandably, the most affected.
+
+Some notable changes below:
+
+| Third-Party | Previously (per-script) | Now (per-page) |
+| ----------- | ----------------------- | -------------- |
+| Beeketing   | 137 ms                  | 465 ms         |
+| Sumo        | 263 ms                  | 798 ms         |
+| Tumblr      | 324 ms                  | 1499 ms        |
+| Yandex APIs | 393 ms                  | 1231 ms        |
+| Google Ads  | 402 ms                  | 1285 ms        |
+| Wix         | 972 ms                  | 5393 ms        |