Replace base::flat_map (sorted vector with O(n) insertion) with
std::unordered_map for CustomEmojiManager::_instances and std::map
for EmojiListWidget::_customEmoji. These maps accumulate ~8000
entries during startup with unsorted keys, causing O(n²) total
insertion cost. This change reduces refreshCustom() from ~3s to
~50ms (57x speedup).
futpib
dc6b9dda4d