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/2019-02-01.md

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+Huge props to [WordAds](https://wordads.co/) for reducing their impact from ~2.5s to ~200ms on average! A few entities are showing considerably less data this cycle (Media Math, Crazy Egg, DoubleVerify, Bootstrap CDN). Perhaps they've added new CDNs/hostnames that we haven't identified or the basket of sites in HTTPArchive has shifted away from their usage.

node_modules/third-party-web/lib/markdown/updates/2019-03-01.md

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+Almost 2,000 entities tracked now across ~3,000+ domains! Huge props to [@simonhearne](https://twitter.com/simonhearne) for making this massive increase possible. Tag Managers have now been split out into their own category since they represented such a large percentage of the "Mixed / Other" category.

node_modules/third-party-web/lib/markdown/updates/2019-05-06.md

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+Google Ads clarified that `www.googletagservices.com` serves more ad scripts than generic tag management, and it has been reclassified accordingly. This has dropped the overall Tag Management share considerably back down to its earlier position.

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

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+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        |

node_modules/third-party-web/lib/markdown/updates/2021-01-01.md

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+Due to a change in HTTPArchive measurement which temporarily disabled site-isolation (out-of-process iframes), all of the third-parties whose work previously took place off the main-thread are now counted _on_ the main thread (and thus appear in our stats). This is most evident in the change to Google-owned properties such as YouTube and Doubleclick whose _complete_ cost are now captured.