msglist: Add double-tap to toggle thumbs up reaction.

[E-m-i-n-e-n-c-e]

This PR adds the ability to double-tap messages to quickly add/remove 👍 reactions

Closes #969.

Video:

20250125-0900-02.5007496.mp4

I placed the double tap gesture in the same gesture detector that detects the long press to open message action sheet. Reused the same logic that is being used to add or remove reactions in message action sheet. So it should handle all cases correctly

      onDoubleTap: () {
        // Access the PerAccountStore to get user-specific data.
        final store = PerAccountStoreWidget.of(context);
        // First emoji in popular Candidates is thumbs up
        final thumbsUpEmoji = EmojiStore.popularEmojiCandidates.toList()[0];
        // Check if the user has already reacted with thumbs up
        // Uses the same logic as hasSelfVote() in action_sheet.dart
        final isSelfVoted = message.reactions?.aggregated.any((reactionWithVotes) =>
          reactionWithVotes.reactionType == ReactionType.unicodeEmoji
          && reactionWithVotes.emojiCode == thumbsUpEmoji.emojiCode
          && reactionWithVotes.userIds.contains(store.selfUserId)) ?? false;

        // Get the localizations for the current context to display appropriate messages.
        final zulipLocalizations = ZulipLocalizations.of(context);

        // Call the function to add or remove the reaction based on the user's current state.
        doAddOrRemoveReaction(
          context: context,
          doRemoveReaction: isSelfVoted,
          messageId: message.id,
          emoji: thumbsUpEmoji,
          errorDialogTitle: isSelfVoted
            ? zulipLocalizations.errorReactionRemovingFailedTitle
            : zulipLocalizations.errorReactionAddingFailedTitle);
      },

Edit:

Added 4 tests under the group 'double-tap gesture' to the message_list_test.dart file

• add thumbs up reaction on double-tap
• remove thumbs up reaction on double-tap when already reacted
• shows error dialog when adding reaction fails
• shows error dialog when removing reaction fails

Again reused the same logic that was being used to test reaction adding and removing in message action sheet ;)