backend:
- add message_hidden table for per-user message hiding
- support DELETE /messages/{id}?for_all=true|false
- implement delete-for-me vs delete-for-all logic by chat type/permissions
- add POST /chats/{chat_id}/clear and route saved chat deletion to clear
web:
- saved messages action changed from delete to clear
- message context menu now supports delete modal: for me / for everyone
- add local store helpers removeMessage/clearChatMessages
- include realtime stability improvements and app error boundary
