Make FeedList slot decisions follow container

FeedList (`apps/web/src/ui/feed/FeedList.tsx`) decides which slots to show from the viewport width. A narrow container inside a wide viewport therefore keeps every slot, and the row overflows and gets clipped. Example: Settings › API keys at 1280px with the company-setup dock restored leaves the list ~400px wide beside the category column. The key name and the row's ⋯ are cut off, and the ⋯ ends up under the dock. - Make FeedList's slot gates follow the list's own width (a container query or a measured width), so a row drops slots instead of clipping in any narrow column. - Then reconsider whether Settings still needs the forced dock rail added in Task 10028. Out of scope: the onboarding dock's open/rail rules, apart from the reconsideration above. Do not touch the column set of any list. Done means: at 1280px on /settings/api-keys with the dock restored, every row's ⋯ is visible and clickable and nothing overflows the list. A Playwright check covers this.

FeedList now drops window-gated slots by the list's own width (ui/feed/fit.ts), with an e2e check at 1280 with the dock restored. Quick gate, vitest and 81 e2e green; PR #6912 queued. The Settings rail is kept: at 1024 the list is ~185px. Deferred: lists narrower than their ungated slots. Memory: none applied.