fix(client): stop demo/RESET from wiping preferences and fix hidden-scenarios toggle

demo/RESET previously reinitialized the entire Redux tree, silently
resetting dev/creator preferences (dark mode, hidden scenarios,
revocation, showcase upload) back to defaults. Preserve the
preferences slice across the reset instead.

Also remove showHiddenScenarios from IntroductionPage's mount-only
redirect/reset effect dependencies, since toggling the preference
was re-triggering that effect and dispatching demo/RESET, undoing
the toggle in the same tick.

Signed-off-by: Emiliano Suñé <2395873+esune@users.noreply.github.com>

Author: esune

frontend/src/client/pages/introduction/IntroductionPage.tsx

@@ -57,7 +57,8 @@ export const IntroductionPage: React.FC = () => {
       dispatch(fetchAllShowcases())
       setMounted(true)
     }
-  }, [dispatch, showHiddenScenarios])
+    // Runs once on mount only; toggling showHiddenScenarios must not re-trigger this redirect/reset
+  }, [dispatch])
 
   useEffect(() => {
     trackPageView()

frontend/src/client/slices/index.ts

@@ -45,7 +45,8 @@ const pReducer = (state: any, action: any) => {
   }
 
   if (action.type === 'demo/RESET') {
-    return rootReducer(undefined, action)
+    // Reset demo progress, but keep dev/creator preferences (dark mode, hidden scenarios, etc.) intact
+    return rootReducer({ preferences: restState.preferences }, action)
   }
 
   return rootReducer(state === undefined ? undefined : restState, action)