TUI: collapse repeated sort-order blocks in JobsData::sort_jobs and share table scrolling helpers #2605

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Two small duplication spots in the ballista-cli TUI domain types:

• JobsData::sort_jobs (tui/domain/jobs.rs#L98-L175) repeats the same let cmp = ...; if sort_order == Descending { cmp.reverse() } else { cmp } block in six match arms. That is why it scores cognitive complexity 42 for what is really six key extractors, and a new column means pasting the block again.
• scroll_down / scroll_up and the get_selected_*_index accessor are copied verbatim across ExecutorsData (executors.rs#L217-L265), JobsData (jobs.rs#L192-L240) and MetricsData (metrics.rs#L77-L124), differing only in the backing collection. The wrap-around and empty-list handling is subtle enough that three copies can drift.

Describe the solution you'd like

• In sort_jobs, match on the column once to pick a comparator closure, then call sort_by once and apply .reverse() for Descending.
• Add a small shared helper (e.g. scroll_down(&mut TableState, &mut ScrollbarState, len) / scroll_up(...), or a ScrollableTable struct) in tui/domain/mod.rs and have the three types delegate to it. Move the scroll tests to the helper.

Describe alternatives you've considered

None.

Additional context

This is small and self-contained, a good first issue. Line references are pinned to commit ea1126c.