feat(dash): scrollbars on scrolling panels, with click-and-drag

[michaelroy-amd]

What

Panels gave no visual sign when their content overflowed, and there was no way to grab a scrollbar with the mouse.

• Reusable panel::vertical_scrollbar / horizontal_scrollbar helpers: no-op when content fits, otherwise draw a bar and return the shrunk content rect.
• Wired into the job console (vertical + horizontal), the LOGS dock, bench detail, chat transcript, and all seven list managers.
• Click-and-drag: each drawn bar records a per-frame hit-test handle; a click on a track jumps to that position and arms a drag that keeps tracking the pointer until release. The tail-anchored LOGS dock offset is inverted so dragging down goes toward older lines.
• Also clamps the job console's horizontal offset against the widest line.

Excluded on purpose: the list managers' bars are drag-to-jump-selection would be awkward (they're driven by ListState selection), so those show a bar but aren't draggable. Easy follow-on.

Tests

• panel::tests: no-op-when-fits, shrink-on-overflow (V and H).
• app::tests: proportional position mapping, full grab → drag → release cycle, dock inversion.
• Full crate suite green (--test-threads=1), clippy -D warnings clean.

Notes

Part 2 of 3. Stacked on #91 (both touch the draw_job_console signature / call site) — please merge #91 first, then this retargets to main.