feat(web): recency headers on Sidebar V2 settled tail

[patroza]

Summary

Phase A of the V2 + Recent integration strategy: add calendar recency headers only on the settled shelf, without changing V2’s lifecycle model.

• Active cards stay creation-order stable (no activity reordering).
• Snoozed shelf unchanged.
• Settled history (already sorted by settle/activity end time) is bucketed with shared thread-recency-groups:
  • Last Hour → Earlier Today → Yesterday → Previous 7/30 Days → Older
• Headers render only when more than one non-empty bucket is visible on the current page (same “no lone header” rule as classic Threads).
• “Show more” paging still applies to the thread list; recency is a presentation layer on the visible page.

Test plan

• Enable Sidebar V2 beta; settle threads across several days → Settled shelf shows date headers.
• All settled rows in the last hour → no section headers under Settled.
• Expand/collapse Settled shelf still works; route-open settled thread still appears when collapsed.
• Show more settled preserves order and updates buckets as more rows appear.
• Active list does not gain recency headers or reorder on activity.
• Sidebar.logic unit tests pass (groupSettledThreadsByRecencyForSidebarV2).