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/has-flag/index.d.ts

@@ -0,0 +1,39 @@
+/**
+Check if [`argv`](https://nodejs.org/docs/latest/api/process.html#process_process_argv) has a specific flag.
+
+@param flag - CLI flag to look for. The `--` prefix is optional.
+@param argv - CLI arguments. Default: `process.argv`.
+@returns Whether the flag exists.
+
+@example
+```
+// $ ts-node foo.ts -f --unicorn --foo=bar -- --rainbow
+
+// foo.ts
+import hasFlag = require('has-flag');
+
+hasFlag('unicorn');
+//=> true
+
+hasFlag('--unicorn');
+//=> true
+
+hasFlag('f');
+//=> true
+
+hasFlag('-f');
+//=> true
+
+hasFlag('foo=bar');
+//=> true
+
+hasFlag('foo');
+//=> false
+
+hasFlag('rainbow');
+//=> false
+```
+*/
+declare function hasFlag(flag: string, argv?: string[]): boolean;
+
+export = hasFlag;