* fix(web): make Done/Terminated dashboard section scrollable (#1923)
The collapsible Done/Terminated cards grid had no max-height or
overflow handling, so it grew unbounded and was clipped by the
ancestor's overflow:hidden once enough terminated sessions
accumulated. The regression came from the Warm Terminal design
refresh (#927) which added `flex: 1` to `.kanban-board-wrap`,
defeating the body-level `overflow-y: auto` that previously
allowed natural page scroll past the done section.
Mirror the existing `.kanban-column-body` pattern by giving the
cards grid its own internal scroll container and matching thin
scrollbar styling. Each scrolling region owns its own scroll;
the Warm Terminal flex layout is preserved.
* style(web): document done-bar__cards max-height magic number
Addresses greptile P2 review feedback on PR #1925. The 320px in
calc(100vh - 320px) is the assumed chrome above the cards
(dashboard header + subhead + body padding + done-bar toggle and
margins). Adding a comment makes the implicit ancestor-height
contract explicit, and points to the kanban-board rule that
uses the same viewport-anchored pattern.
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