frontend: add dev debugger dialog

Adds a dev-only debugging dialog with connect-config fixtures and
feature-flag overrides. Wired into the root layout (dev builds only)
and app setup.

Moved here from the UX-1275/template-gallery-basic branch to keep that
PR smaller and more focused.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: SpicyPete

frontend/src/app.tsx

@@ -48,6 +48,7 @@ import { useEffect } from 'react';
 import { getBasePath } from 'utils/env';
 import { patchedRedpandaTheme as redpandaTheme } from 'utils/redpanda-theme';
 
+import { applyOverrides as applyDebugFeatureFlagOverrides } from './components/debug-helper/feature-flag-overrides';
 import { NotFoundPage } from './components/misc/not-found-page';
 import { addBearerTokenInterceptor, checkExpiredLicenseInterceptor, getGrpcBasePath, setup } from './config';
 import { routeTree } from './routeTree.gen';
@@ -106,6 +107,9 @@ const App = () => {
 
   useEffect(() => {
     const setupTeardown = setup(EMPTY_SETUP_ARGS);
+    // Apply local debug feature-flag overrides AFTER setup() (which clobbers
+    // config.featureFlags). Dev-only mechanism — see debug-helper/feature-flag-overrides.ts.
+    applyDebugFeatureFlagOverrides();
     const uiSettingsTeardown = installUISettingsSideEffects();
     return () => {
       uiSettingsTeardown();