Re-measure timeline row when a collapsed event group expands

[rexbron]

Fixes #150 — expanding a collapsed list of call/system events would clip the content and push the timeline off screen.

Cause

The table-backed timeline renderer caches row heights keyed by (messageID, width). Expanding a collapsed group only flips the shared @Observable ExpandedGroupsState — it doesn't change the rows data, so updateRows never diffs that row, the height cache keeps the collapsed value, and heightOfRow returns it. NSTableView then clips the taller expanded content to the old height, and the un-accounted-for growth throws off scroll position.

Fix

• ExpandedGroupsState.toggle now fires an onToggle(groupID) callback after mutating.
• The table controller's new remeasureRow(forMessageID:) invalidates that row's cached height and calls noteHeightOfRows. heightOfRow's measurement host rebuilds the row reading the updated expansion state, so it returns the full expanded (or collapsed) height.
• Scroll position is preserved across the height change, so expanding a row above the viewport no longer shifts the visible content off screen.
• The representable wires expandedGroups.onToggle to the controller.

The LazyVStack renderer auto-sizes and was never affected — this is specific to the table renderer's height cache.

Testing

• Expand/collapse a collapsed group of call/membership events → the row grows/shrinks to fit instead of clipping, and the timeline stays put.