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/settings/search.ts

@@ -240,6 +240,19 @@ const SETTINGS_SEARCH_ITEMS: readonly SettingsSearchItem[] = [
     titleKey: 'settings.openchamber.visual.section.reasoning',
     keywords: ['thinking', 'traces'],
   },
+  {
+    id: 'chat.streaming',
+    page: 'chat',
+    titleKey: 'settings.openchamber.visual.section.streaming',
+    keywords: ['stream', 'scroll'],
+  },
+  {
+    id: 'chat.streaming-auto-follow',
+    page: 'chat',
+    titleKey: 'settings.openchamber.visual.field.streamingAutoFollow',
+    descriptionKey: 'settings.openchamber.visual.field.streamingAutoFollowInfo',
+    keywords: ['autoscroll', 'auto-scroll', 'follow', 'stick to bottom', 'streaming'],
+  },
   {
     id: 'chat.sticky-user-header',
     page: 'chat',