add icon, also misc improvments

lib/helpers/reaction_helper.dart

@@ -0,0 +1,22 @@
+class ReactionInfo {
+  final String targetMessageId;
+  final String emoji;
+
+  ReactionInfo({
+    required this.targetMessageId,
+    required this.emoji,
+  });
+}
+
+class ReactionHelper {
+  /// Parse reaction format: r:[messageId]:[emoji]
+  static ReactionInfo? parseReaction(String text) {
+    final regex = RegExp(r'^r:([^:]+):(.+)$');
+    final match = regex.firstMatch(text);
+    if (match == null) return null;
+    return ReactionInfo(
+      targetMessageId: match.group(1)!,
+      emoji: match.group(2)!,
+    );
+  }
+}