Enable React Router v7 future flags to silence warnings in tests

Added future flags to BrowserRouter in the dummy app to enable v7-compatible
behavior and eliminate console warnings that were causing test failures.

Changes:
- spec/dummy/client/app/startup/RouterApp.client.jsx: Added future prop with
  v7_startTransition and v7_relativeSplatPath flags

Why this fixes the issue:
The dummy app integration tests were failing because selenium_logger.rb treats
JavaScript console warnings as errors. React Router v6 emits warnings about
upcoming v7 changes:

  ⚠️ React Router Future Flag Warning: React Router will begin wrapping state
  updates in React.startTransition in v7. You can use the v7_startTransition
  future flag to opt-in early.

  ⚠️ React Router Future Flag Warning: Relative route resolution within Splat
  routes is changing in v7. You can use the v7_relativeSplatPath future flag
  to opt-in early.

By enabling these flags, we:
1. Silence the warnings
2. Adopt v7-compatible behavior early
3. Demonstrate best practices for React Router v6 users
4. Align with React Router's official migration guide

Reference: https://reactrouter.com/upgrading/future

Fixes test failures:
- React Router when rendering /react_router is expected to have text "Woohoo..."
- React Router when rendering /react_router clicking links correctly renders...

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ihabadham

spec/dummy/client/app/startup/RouterApp.client.jsx

@@ -2,4 +2,14 @@ import React from 'react';
 import { BrowserRouter } from 'react-router-dom';
 import routes from '../routes/routes';
 
-export default (props) => <BrowserRouter {...props}>{routes}</BrowserRouter>;
+export default (props) => (
+  <BrowserRouter
+    {...props}
+    future={{
+      v7_startTransition: true,
+      v7_relativeSplatPath: true,
+    }}
+  >
+    {routes}
+  </BrowserRouter>
+);