feat: Add browser-like back navigation and link replay for --ui

[BlueLabelscotch]

Summary

--ui mode could only go forward — every link/up/view hop discarded the
running program with no way back, so q deep in a chain exited the whole
process. This adds real back-navigation:

• New cmdctx.UILink + dispatchLink replay mechanism, re-resolving auth
  from the raw scope strings captured at hop time.
• Session-lifetime UIHistory stack, pushed on every hop, popped by a new
  b key to redraw the previous screen.
• Exiting a view screen (e.g. log viewer) now resumes where it was
  launched from, instead of exiting the session.
• b inside an in-place list→detail flip now collapses like esc, instead
  of touching history.
• List cursor position and active search filters are captured on hop-leave
  and restored on replay.