Add 'Move to Active' button on completed sessions

[dhilgaertner]

Description

When viewing a session that is in the completed state, add a button next to the existing Open in VS Code button that allows the user to move the session back into the active state.

Why

Sometimes a session gets marked completed prematurely, or there's follow-up work that warrants resuming it rather than starting a new session. Currently there's no quick way to reactivate a completed session from the UI.

Proposed UX

• Location: session detail view, adjacent to the existing Open in VS Code button.
• Label: something like Move to Active (or Reactivate).
• Behavior: clicking sets the session status to active (equivalent to crow set-status --session <uuid> active) and updates the UI accordingly.
• Visibility: only show when the current session status is completed.