feat: show model in working indicator and fix streaming bottom jitter

- Working indicator shows provider icon and model name with the live
  status (e.g. 'Fable 5 is reading file'), localized via
  chat.statusRow.modelStatus
- Pin auto-follow to the exact fractional scroll maximum and re-pin on
  every passive follow instead of skipping within tolerance
- Round message text line-height to whole pixels so streamed content
  grows on the pixel grid; kills the 1px vertical jitter of
  bottom-anchored rows during streaming

packages/ui/src/lib/i18n/messages/ja.ts

@@ -1793,6 +1793,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.statusRow.todo.priority.low': '低優先度',
   'chat.statusRow.actions.stopGeneratingAria': '生成を停止',
   'chat.statusRow.tasksTitle': 'タスク',
+  'chat.statusRow.modelStatus': '{model} · {status}',
   'chat.statusRow.summary.activeLeft': '{active}アクティブ · {left}残り',
   'chat.statusRow.aborted': '中止されました',
   'chat.revertIndicator.redo': 'やり直し',