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/components/chat/Chat.tsx

@@ -94,12 +94,7 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
   // Expose methods to parent via ref
   useImperativeHandle(ref, () => ({
     loadConversation: async (cId: string): Promise<void> => {
-      try {
-        await loadConversation(cId);
-      } catch (err) {
-        console.error("Failed to load conversation", { error: err, conversationId: cId });
-        throw err;
-      }
+      await loadConversation(cId);
     },
     startNewConversation: (projectId?: string | null): void => {
       startNewConversation(projectId);