refactoring: switched to gpt-5-nano for commit message generation

Remove commitMessageModel from persisted settings and defaults
Eliminate UI controls and state for commitMessageModel in GitSettings and config store
Update backend and persistence layers to stop reading or writing commitMessageModel

packages/ui/src/hooks/useChatScrollManager.ts

@@ -374,7 +374,7 @@ export const useChatScrollManager = ({
                     ? (container.querySelector(`[data-message-id="${persistedAnchor.anchorId}"]`) as HTMLElement | null)
                     : null;
                 const messageHeight = anchorElement?.offsetHeight ?? 0;
-                const restoredSpacerHeight = Math.max(0, persistedAnchor.spacerHeight - messageHeight);
+                const restoredSpacerHeight = Math.max(0, persistedAnchor.spacerHeight - (messageHeight - 50));
 
                 flushSync(() => {
                     setAnchorId(persistedAnchor.anchorId);