Implement Collapsible/Tabbed Right Pane on Mobile

[bryanngongeh]

Modify App.tsx to conditionally render a collapsible or tabbed right pane (Plan | Chat) on phones, while retaining the full side-by-side layout for widths 1024px and above.

• Acceptance Criteria:

• On phone screen sizes, the Plan and Chat panels are presented in a tabbed or collapsible interface.

• Users can easily switch between the Plan and Chat views.

• On larger screens (≥1024px), the Plan and Chat panels are displayed side-by-side.