feat(chat): message grouping, day separators, quick-reaction row (v1.9.0+13)

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>

pubspec.yaml

@@ -1,7 +1,7 @@
 name: m8chat_app
 description: "M8Chat — Matrix chat client for Android, iOS, and Web."
 publish_to: 'none'
-version: 1.8.0+12
+version: 1.9.0+13
 
 environment:
   sdk: '>=3.11.0 <4.0.0'