fix(#183): "b" back key does nothing in the execution log viewer

[BlueLabelscotch]

Pressing b in the --ui execution log viewer (v from an execution row) did nothing — it's a standalone tea.Program that never got wired into the app-wide b-for-back convention.

• logviewer.go: b now cancels streams and quits, same as q.
• cmdctx.go: added UIWantBack (default false) so a view-handler can opt into "go back" instead of "quit".
• uitableview.go: finishUIExit now only pops/redraws history when UIWantBack is set.
• Help footer now shows b back.

fix for #183

[BlueLabelscotch]

@sawka-harness , ctx.Resolver.RunUIHandler(ctx, uc.UIHandlerFn) calls handler functions and never gets a logViewModel back, so we can't structurally know if the viewer decided to go 'b' back or 'q' quit , needed to add a common flag in the ctx, which will be changed inside the log viewer to carry that info to the finishUiexit, that will solve our log view problem.