fix: show alert instead of redirecting to non-existent /client route

When profile guard fails (single-player profile detected):
- Show JavaScript alert explaining the issue
- Leave session cleanly via handleLeaveSession()
- Redirect to dashboard (/) instead of /client (404)

Alert message explains:
- Audio profile not suitable for multi-user sessions
- Need proper audio interface or create private session

TODO: Replace alert() with proper modal dialog component
that offers options like legacy app (create private session,
go to audio settings, cancel).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

jam-ui/src/components/client/JKSessionScreen.js

@@ -376,10 +376,17 @@ const JKSessionScreen = () => {
 
             } catch (error) {
               logger.error("User profile is not appropriate for session:", error);
-              // If error doesn't control navigation, redirect to home (legacy behavior)
+              // If error doesn't control navigation, show alert (legacy behavior shows dialog)
               if (!error?.controlled_location) {
-                logger.debug("Profile guard failed, redirecting to home");
-                history.push('/client');
+                logger.debug("Profile guard failed, showing alert");
+                // TODO: Replace with proper JKSessionProfileDialog component
+                alert(
+                  'Your current audio profile is not suitable for multi-user sessions.\n\n' +
+                  'Please configure a proper audio interface in your audio settings, ' +
+                  'or create a private session for solo practice.'
+                );
+                await sessionModel.handleLeaveSession();
+                history.push('/'); // Redirect to dashboard
                 return; // Don't continue with session join
               }
               // If controlled_location is true, the error handler already redirected