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/desktop.ts

@@ -131,6 +131,7 @@ export type DesktopSettings = {
   defaultVariant?: string;
   defaultAgent?: string;
   smallModelUseDefault?: boolean;
+  streamingAutoFollowEnabled?: boolean;
   sessionRecapEnabled?: boolean;
   sessionSuggestionEnabled?: boolean;
   sessionGoalEnabled?: boolean;