Don't cancel stream on navigation away

Aborting the fetch on component unmount sent a disconnect to the server,
which cancelled the in-flight generation. Now the stream runs to
completion in the background (isMountedRef guards prevent stale state
updates), the response is persisted, and it's there when the user
returns to the conversation. stopGeneration() still aborts immediately
on explicit user cancel.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

raggr-frontend/src/hooks/useChat.ts

@@ -67,7 +67,9 @@ export function useChat({
     isMountedRef.current = true;
     return () => {
       isMountedRef.current = false;
-      abortControllerRef.current?.abort();
+      // Intentionally do NOT abort on unmount — navigation away lets the
+      // stream finish in the background so the response gets persisted.
+      // stopGeneration() still aborts on explicit user cancel.
     };
   }, []);