B1 grouping: same-sender runs within 5 min collapse to one name + one
avatar (last-of-run), tight padding, tail corner only on last bubble
B2 day separators: Today/Yesterday/d MMM chip above each local day's
first message, Column wraps the gesture widget (long-press preserved)
B4 quick reactions: six preset emoji + full-picker button atop the
long-press menu, wired to the existing sendReaction provider
Verified end-to-end in headless Chromium on the production build;
tapping a preset sends a real m.reaction (confirmed server-side).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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