Preview selected session pane

[semantic-craft]

Parent

Parent: #2

What to build

Add a lightweight Dashboard preview for the selected tmux session's active pane. The user should be able to inspect recent terminal output, active command, and current directory before switching to or acting on a session.

The preview should reuse existing tmux capture behavior and remain modest: refresh on selection/open/manual refresh, not continuous streaming.

Acceptance criteria

• Selecting a session in the Dashboard shows a preview of that session's active pane.
• If no active pane is known, the preview falls back to the first pane in the session.
• If the session has no panes, the preview shows a clear empty/fallback state.
• The preview includes recent pane output, current command, and current directory when available.
• Capture failures show a contained fallback message and do not break the Dashboard.
• Preview refresh happens on selection/open/manual refresh, without adding continuous streaming.
• Preview selection and failure behavior are covered by focused tests at the shared state/service seam.
• Manual verification includes selecting a running session, a quiet shell session, and a killed/changed session.

Blocked by

None - can start immediately