Have id field on ModelItem base class

TODO.md

@@ -3,6 +3,7 @@
 - add account number binds
 - rename goto*account → scrollto*account
 
+- fix opacity
 - fix python error
 - fix back/front buttons in small window
 - fix message delegate too tall
@@ -17,6 +18,9 @@
 
 - if last room event is a membership change, it won't be visible in timeline
 - use uiState instead of open_room
+- clicking on a room with unread counter to see it move right away is weird
+- rooms without messages on first sync
+- avatar loading performance problem?
 
 ## Refactoring
 
@@ -35,8 +39,6 @@
 
 - Drag-scrolling in room pane a tiny bit activates the delegates
 
-- When zooming in enough, "Mirage <version>" label overflows
-
 - Catch server 5xx errors when sending message and retry 
 
 - Popups and room settings can't be scrolled when not enough height to show all
@@ -44,9 +46,6 @@
 - Handle cases where a known account's access token is invalid
 - If an account is gone from the user's config, discard UI state last page
 
-- First sent message in E2E room is sometimes undecryptable
-  (can be fixed by logging out and in again) (still valid?)
-
 - After forgetting a room, it comes back because of the "you left" event
 
 - `code` and links in quote ("> http://example.com") aren't properly colored

src/backend/models/model_item.py

@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
+from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any, Dict
 
 from ..pyotherside_events import ModelItemSet
@@ -10,6 +11,7 @@ if TYPE_CHECKING:
     from .model import Model
 
 
+@dataclass
 class ModelItem:
     """Base class for items stored inside a `Model`.
 
@@ -21,6 +23,9 @@ class ModelItem:
     and thus allow a `Model` to keep its data sorted.
     """
 
+    id: Any = field()
+
+
     def __new__(cls, *_args, **_kwargs) -> "ModelItem":
         cls.parent_models: Dict[SyncId, Model] = {}
         return super().__new__(cls)