feat: optimize reaction message format to reduce airtime

- Reduce reaction payload from ~44 bytes to 9 bytes (5x smaller)
- Use 4-char hex hash (timestamp + sender + first 5 chars) for message ID
- Use 2-char hex emoji index instead of multi-byte UTF-8 emoji
- Format: r:HASH:INDEX (e.g., r:a1b2:00)
- For 1:1 chats, sender is implicit (null) for shorter hash
- Prevent users from reacting to their own messages
- Add room server reaction support with sender identification
- Make emoji lists public in EmojiPicker for shared indexing
- Add 💪 and 🚀 emojis to picker
- Add comprehensive unit tests for reaction helpers
- Update minor dependencies

lib/helpers/reaction_helper.dart

@@ -1,53 +1,70 @@
+import '../widgets/emoji_picker.dart';
+
 class ReactionInfo {
-  final String targetMessageId;
+  final String targetHash;
   final String emoji;
-  final String? reactionKey; // Lightweight key for deduplication: timestamp_senderPrefix
 
   ReactionInfo({
-    required this.targetMessageId,
+    required this.targetHash,
     required this.emoji,
-    this.reactionKey,
   });
 }
 
 class ReactionHelper {
-  /// Parse reaction format: r:[messageId]:[emoji]
-  /// Supports both old format (full messageId) and new format (timestamp_senderPrefix)
+  static List<String>? _cachedEmojis;
+
+  /// Combined list of all reaction emojis in fixed order.
+  /// Order must stay stable for index compatibility.
+  static List<String> get reactionEmojis {
+    return _cachedEmojis ??= [
+      ...EmojiPicker.quickEmojis,
+      ...EmojiPicker.smileys,
+      ...EmojiPicker.gestures,
+      ...EmojiPicker.hearts,
+      ...EmojiPicker.objects,
+    ];
+  }
+
+  /// Convert emoji to 2-char hex index. Returns null if emoji not in list.
+  static String? emojiToIndex(String emoji) {
+    final idx = reactionEmojis.indexOf(emoji);
+    if (idx < 0) return null;
+    return idx.toRadixString(16).padLeft(2, '0');
+  }
+
+  /// Convert 2-char hex index to emoji. Returns null if invalid index.
+  static String? indexToEmoji(String hexIndex) {
+    final idx = int.tryParse(hexIndex, radix: 16);
+    if (idx == null || idx < 0 || idx >= reactionEmojis.length) return null;
+    return reactionEmojis[idx];
+  }
+
+  /// Compute a 4-char hex hash for a message reaction.
+  /// Hash input: timestampSeconds + [senderName] + first 5 chars of text
+  /// For 1:1 chats, senderName can be null (sender is implicit).
+  static String computeReactionHash(int timestampSeconds, String? senderName, String text) {
+    final first5 = text.length >= 5 ? text.substring(0, 5) : text;
+    final input = senderName != null
+        ? '$timestampSeconds$senderName$first5'
+        : '$timestampSeconds$first5';
+    // Use hashCode and take lower 16 bits, format as 4 hex chars
+    final hash = input.hashCode & 0xFFFF;
+    return hash.toRadixString(16).padLeft(4, '0');
+  }
+
+  /// Parse reaction format: r:HASH:INDEX (where INDEX is 2-char hex emoji index)
+  /// Returns null if text is not a valid reaction format
   static ReactionInfo? parseReaction(String text) {
-    final regex = RegExp(r'^r:([^:]+):(.+)$');
+    final regex = RegExp(r'^r:([0-9a-f]{4}):([0-9a-f]{2})$');
     final match = regex.firstMatch(text);
     if (match == null) return null;
 
-    final targetId = match.group(1)!;
-    final emoji = match.group(2)!;
-
-    // Extract reaction key for deduplication
-    // If targetId is in new format (timestamp_senderPrefix), use it directly
-    // Otherwise, extract timestamp from old format (timestamp_nameHash_textHash)
-    String? reactionKey;
-    if (targetId.contains('_')) {
-      final parts = targetId.split('_');
-      if (parts.length >= 2) {
-        // New format: timestamp_senderPrefix, or old format with at least timestamp
-        reactionKey = '${parts[0]}_${parts[1]}';
-      }
-    }
+    final emoji = indexToEmoji(match.group(2)!);
+    if (emoji == null) return null;
 
     return ReactionInfo(
-      targetMessageId: targetId,
+      targetHash: match.group(1)!,
       emoji: emoji,
-      reactionKey: reactionKey,
     );
   }
-
-  /// Generate a lightweight reaction key for a message
-  /// Format: r:[timestamp]_[senderPrefix]:[emoji]
-  static String buildReactionText(String timestamp, String senderPrefix, String emoji) {
-    return 'r:${timestamp}_$senderPrefix:$emoji';
-  }
-
-  /// Extract sender prefix from public key hex (first 8 chars)
-  static String getSenderPrefix(String senderKeyHex) {
-    return senderKeyHex.substring(0, 8);
-  }
 }