fix(chat): anchor prompt navigator to last turn at chat bottom (#2213)

* fix(chat): anchor prompt navigator to last turn at chat bottom

The scroll spy activates the last turn whose top edge crossed the reading
line (100px below the container top). A final turn short enough to fit
below that line could never become active — sitting at the very bottom of
the chat kept the previous prompt highlighted on the navigator rail.
When the container is scrolled to the bottom (within 8px), force the last
turn active.

* feat(chat): enable prompt navigator by default

Users who already persisted an explicit preference keep their choice.

packages/ui/src/stores/useUIStore.ts

@@ -964,7 +964,7 @@ export const useUIStore = create<UIStore>()(
         userMessageRenderingMode: 'markdown',
         collapsibleUserMessages: true,
         stickyUserHeader: false,
-        promptNavigatorEnabled: false,
+        promptNavigatorEnabled: true,
         expandedEditorToolbar: false,
         showSplitAssistantMessageActions: false,
         allowPromptingSubagentSessions: false,