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/lib/persistence.ts

@@ -531,6 +531,7 @@ const materializeAuthoritativeUiSettings = (settings: DesktopSettings): DesktopS
     darkThemeId: DEFAULT_DARK_THEME_ID,
     openInAppId: DEFAULT_OPEN_IN_APP_ID,
     showReasoningTraces: defaults.showReasoningTraces,
+    streamingAutoFollowEnabled: defaults.streamingAutoFollowEnabled,
     workStatusPanelEnabled: defaults.workStatusPanelEnabled,
     workStatusHiddenSections: defaults.workStatusHiddenSections,
     sessionRecapEnabled: defaults.sessionRecapEnabled,
@@ -637,6 +638,9 @@ const applyDesktopUiPreferences = (settings: DesktopSettings) => {
   if (typeof settings.showReasoningTraces === 'boolean' && settings.showReasoningTraces !== store.showReasoningTraces) {
     store.setShowReasoningTraces(settings.showReasoningTraces);
   }
+  if (typeof settings.streamingAutoFollowEnabled === 'boolean' && settings.streamingAutoFollowEnabled !== store.streamingAutoFollowEnabled) {
+    store.setStreamingAutoFollowEnabled(settings.streamingAutoFollowEnabled);
+  }
   if (typeof settings.sessionRecapEnabled === 'boolean' && settings.sessionRecapEnabled !== store.sessionRecapEnabled) {
     store.setSessionRecapEnabled(settings.sessionRecapEnabled);
   }
@@ -1158,6 +1162,9 @@ const sanitizeWebSettings = (payload: unknown): DesktopSettings | null => {
   if (typeof candidate.showReasoningTraces === 'boolean') {
     result.showReasoningTraces = candidate.showReasoningTraces;
   }
+  if (typeof candidate.streamingAutoFollowEnabled === 'boolean') {
+    result.streamingAutoFollowEnabled = candidate.streamingAutoFollowEnabled;
+  }
   if (typeof candidate.sessionRecapEnabled === 'boolean') {
     result.sessionRecapEnabled = candidate.sessionRecapEnabled;
   }