feat(core): clear saved chat and add message deletion scopes

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

app/chats/router.py

@@ -17,6 +17,7 @@ from app.chats.schemas import (
 from app.chats.service import (
     add_chat_member_for_user,
     create_chat_for_user,
+    clear_chat_for_user,
     delete_chat_for_user,
     discover_public_chats_for_user,
     ensure_saved_messages_chat,
@@ -182,6 +183,15 @@ async def delete_chat(
     await delete_chat_for_user(db, chat_id=chat_id, user_id=current_user.id, payload=ChatDeleteRequest(for_all=for_all))
 
 
+@router.post("/{chat_id}/clear", status_code=status.HTTP_204_NO_CONTENT)
+async def clear_chat(
+    chat_id: int,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+) -> None:
+    await clear_chat_for_user(db, chat_id=chat_id, user_id=current_user.id)
+
+
 @router.post("/{chat_id}/pin", response_model=ChatRead)
 async def pin_chat_message(
     chat_id: int,