Migrate the transfer progress UI to tuirealm 4, where the stdlib
`ProgressBar` widget was dropped, by rebuilding the dual-bar panel on
top of `Gauge`.
- Restore the unified two-bar look: the full bar (top) and partial bar
(bottom) draw joined borders so they read as a single panel; a single
file shows one fully-bordered bar.
- Redraw on every file boundary in the send/recv queue loops so the
full bar's (N/total) counter advances even for small files that finish
within one in-loop redraw interval.
- Track progress with a single `TransferProgress` (exact file count from
the pre-scan, lazy partial/full computation) and consolidate the theme
progress-bar fields.