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>

lib/core/config/app_config.dart

@@ -11,7 +11,7 @@ abstract final class AppConfig {
     'turns:matrix.m8chat.au:5349',
   ];
   static const String appName = 'M8Chat';
-  static const String appVersion = '1.8.0';
+  static const String appVersion = '1.9.0';
 
   // Jitsi conferencing
   static const String jitsiDomain = 'conf.m8chat.au';