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.test.ts

@@ -377,6 +377,44 @@ describe("useChat", () => {
       expect(result.current.messages).toHaveLength(1);
       expect(result.current.messages[0]?.id).toBe("welcome");
     });
+
+    it("should set sanitized error and call onError when getIdea rejects", async () => {
+      const consoleSpy = vi.spyOn(console, "error").mockImplementation(() => undefined);
+      mockGetIdea.mockRejectedValueOnce(new Error("Not found"));
+
+      const onError = vi.fn();
+      const { result } = renderHook(() => useChat({ onError }));
+
+      await act(async () => {
+        await result.current.loadConversation("conv-missing");
+      });
+
+      expect(result.current.error).toBe("Unable to load conversation. Please try again.");
+      expect(onError).toHaveBeenCalledWith(expect.any(Error));
+      expect(consoleSpy).toHaveBeenCalledWith(
+        "Failed to load conversation",
+        expect.objectContaining({
+          errorType: "LOAD_ERROR",
+          ideaId: "conv-missing",
+          timestamp: expect.any(String) as string,
+        })
+      );
+      expect(result.current.isLoading).toBe(false);
+    });
+
+    it("should not re-throw when getIdea rejects", async () => {
+      vi.spyOn(console, "error").mockImplementation(() => undefined);
+      mockGetIdea.mockRejectedValueOnce(new Error("Server error"));
+
+      const { result } = renderHook(() => useChat());
+
+      // Should resolve without throwing - errors are handled internally
+      await act(async () => {
+        await expect(result.current.loadConversation("conv-err")).resolves.toBeUndefined();
+      });
+
+      expect(result.current.error).toBe("Unable to load conversation. Please try again.");
+    });
   });
 
   describe("startNewConversation", () => {

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);
       }