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/clipboard/.github/ISSUE_TEMPLATE/proposal.md

@@ -0,0 +1,26 @@
+---
+name: 💥 Proposal
+about: Propose a non-trivial change to Clipboard.js
+labels: 'proposal, needs triage'
+---
+
+## 💥 Proposal
+
+**Is your feature request related to a problem? Please describe**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Are you able to assist to bring the feature to reality?**
+no | yes, I can...
+
+**Additional context**
+Add any other context or screenshots about the feature request here.
+
+### Have you read the [Contributing Guidelines on issues](https://github.com/zenorocha/clipboard.js/blob/master/contributing.md)?
+
+(Write your answer here.)