Add ability to search messages within a specific conversation. This includes:
- New `MessageSearchScreen` for displaying search results.
- `MessageRepository.searchByConversation` to query messages by keyword and conversation ID.
- Integration of search entry points in `GroupInfoScreen` and `PrivateChatInfoScreen`.
- Support for scrolling to a specific message sequence (`scrollToSeq`) when navigating from search results.
- Enhanced `HighlightText` component to support custom highlight styles.
feat(message): implement message search functionality
