fix(web): Remove re-throw from loadConversation to prevent unhandled rejections

- Make loadConversation fully self-contained like sendMessage (handle
  errors internally via state, onError callback, and structured logging)
- Remove duplicate try/catch+log from Chat.tsx imperative handle
- Replace re-throw tests with delegation and no-throw tests
- Add hook-level loadConversation error path tests (getIdea rejection)

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

apps/web/src/hooks/useChat.ts

@@ -290,7 +290,6 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
           ideaId,
           timestamp: new Date().toISOString(),
         });
-        throw err;
       } finally {
         setIsLoading(false);
       }