Add stacked pull request / gh stack view

Is your feature request related to a problem? Please describe.
gh-dash does not show the relationship between PRs in a stack. To see the stack order and dependencies, users must leave the dashboard and run gh stack view or inspect PR base branches manually.

Describe the solution you'd like
Add a stacked-PR view that shows the current stack as an ordered list or tree:

• Parent/child PR relationships
• PR number, title, branch, and status
• Review and CI status
• Navigation and checkout actions for each stack layer

It could use data from gh stack view --json when GitHub Stacked PRs is available.

Describe alternatives you've considered

• Run gh stack view in another terminal pane
• Use GitHub’s web UI
• Infer the stack manually from PR base branches

Additional context
A native stack view would let users review and monitor dependent PRs without leaving gh-dash.