feat(remote): show SSH session output in remote preview

[aaronflorey]

Summary

• fetch the latest remote session output over SSH and render it in the remote preview pane so remote selections have useful context before attach
• cache remote previews with a longer TTL and treat failed or empty fetches as completed attempts, which avoids repeated SSH retries and fixes the stuck loading state for empty responses
• add SSH and UI regression tests covering remote preview fetching, empty-state rendering, and preview cache timestamp handling

Testing

• rtk go test ./internal/ui
• rtk go test ./... (still failing after rebasing onto upstream/main in existing tmux/session/integration tests, including internal/tmux, internal/session, internal/integration, and internal/tuitest)

[asheshgoplani]

Hey @aaronflorey — I've been working through the PR queue tonight and merged your #581, #583, #591 (thanks!). This one (#577) is clean but now has merge conflicts because other PRs touching internal/ui/home.go landed ahead of it. Would you mind rebasing onto current main? The SSH preview work is a valuable addition and I'd like to land it — just need the conflict resolved. Appreciate the patience.