Test: cover frontend error states for dashboard panels

[franklinzerocr]

The current frontend tests focus on the happy path for the Dashboard page and live market price widget.

To better demonstrate robustness, add test coverage for error scenarios:

• When the backend fails for portfolio/signals/ops:
  • Dashboard should show the corresponding error messages.
• When the market price endpoint fails:
  • The widget should render the "Using last known value" copy when a previous value exists.
  • It should render an explicit "Error" state when no previous value is available.

These tests will better showcase how the UI behaves under degraded conditions.