fix(web): keep the compaction notice visible above monitoring - #10182

1. browse_code — apps/web/src/components/chat/ComposerBannerStack.tsx

💭 The new code reuses the existing banner alert and slot components properly, with no raw controls or inline styles, and classes match the existing stacked items pattern. The early return logic changed to check for a front item instead of empty array length, which is a functional tweak rather than a styling concern, and the minor markup duplication stays within the same component. Everything looks clear. Now I need to call complete_check and ensure my final response is exactly "All clear" — I'll use that as the title since it meets the minimum character requirement.