feat(settings): add streaming auto-follow toggle

New Streaming section on the Chat settings page with a checkbox that
controls whether the viewport follows new content while a response
streams. Default stays on. With it off, the anchored user message still
parks at the top on send, but no glide or end-follow correction runs and
the list's maintain-scroll-at-end stays disabled; the scroll-to-bottom
pill and session open keep scrolling explicitly.

Persisted through desktop settings like the other chat toggles (auto-save
diff, authoritative apply, sanitize), registered in settings search, and
localized in every locale.

packages/ui/src/stores/useUIStore.ts

@@ -674,6 +674,7 @@ interface UIStore {
   eventStreamStatus: EventStreamStatus;
   eventStreamHint: string | null;
   showReasoningTraces: boolean;
+  streamingAutoFollowEnabled: boolean;
   sessionRecapEnabled: boolean;
   sessionSuggestionEnabled: boolean;
   sessionGoalEnabled: boolean;
@@ -859,6 +860,7 @@ interface UIStore {
   setSettingsRemoteInstancesSelectedId: (instanceId: string | null) => void;
   setEventStreamStatus: (status: EventStreamStatus, hint?: string | null) => void;
   setShowReasoningTraces: (value: boolean) => void;
+  setStreamingAutoFollowEnabled: (value: boolean) => void;
   setSessionRecapEnabled: (value: boolean) => void;
   setSessionSuggestionEnabled: (value: boolean) => void;
   setSessionGoalEnabled: (value: boolean) => void;
@@ -1027,6 +1029,7 @@ export const useUIStore = create<UIStore>()(
         eventStreamStatus: 'idle',
         eventStreamHint: null,
         showReasoningTraces: true,
+        streamingAutoFollowEnabled: true,
         sessionRecapEnabled: true,
         sessionSuggestionEnabled: true,
         sessionGoalEnabled: true,
@@ -1759,6 +1762,10 @@ export const useUIStore = create<UIStore>()(
           set({ showReasoningTraces: value });
         },
 
+        setStreamingAutoFollowEnabled: (value) => {
+          set({ streamingAutoFollowEnabled: value });
+        },
+
         setSessionRecapEnabled: (value) => {
           set({ sessionRecapEnabled: value });
         },
@@ -2639,6 +2646,7 @@ export const useUIStore = create<UIStore>()(
           isSessionCreateDialogOpen: state.isSessionCreateDialogOpen,
           // Note: isSettingsDialogOpen intentionally NOT persisted
           showReasoningTraces: state.showReasoningTraces,
+          streamingAutoFollowEnabled: state.streamingAutoFollowEnabled,
           sessionRecapEnabled: state.sessionRecapEnabled,
           sessionSuggestionEnabled: state.sessionSuggestionEnabled,
           sessionGoalEnabled: state.sessionGoalEnabled,