fix(frontend): the layer gates cover what the move left outside them, and each page keeps its own logic

Gates the folder move had narrowed or left open: the MenuCheck contract scans the whole app again (it
saw only components/); local/layers refuses an import of an entry (main.tsx, the /watch page) from
any layer, and reads a backtick import() as the path it is; the /watch contract forbids the shell,
the modules and the features by folder (its one path to App had gone stale); chunkGuard's own test
gets the vitest focus/skip rules. The root tests' exemption from local/layers is in the gate
register, with its three imports.

Placement: Plex's chip builder moves into lib beside the feed's, the folder names into
features/folders, and three files only one page uses into that page. ui/FilterRail takes its reset
from useRail instead of a page id. The channel page reads the feed's view and builds "Show in feed"
itself, and the feed's scroll key is the feed's hook, so the shell holds no feed logic.

The floating header re-measures when one page's pills replace another's in the same commit (the
claim count stayed equal, so only the observer caught it). A Plex view applied and edited now stays
across a visit to another page, as the feed's does: pinned by a test. docs/development.md says what a
page with a rail adds in lib.

docs/development.md

@@ -51,6 +51,11 @@ A frontend page is one folder, `frontend/src/modules/<id>/` (the layering is in
 4. The id in `lib/urlState`'s `PAGES`, and the definition in `features/navigation/moduleRegistry.ts`, in
    nav-rail order. The rail, the palette, the `g` chords, the tab title and the root's stores all read from there.
 5. Its strings in both locales (`src/i18n/locales`).
+6. A page with a side rail also names its panel below the module, in `lib/` (rails share one layout
+   and collapse machinery, which lib may not learn from a module): the `PanelId` and its default group
+   order, storage and pref keys in `lib/panelLayout.ts`, its collapse flag in `lib/useRail.ts`
+   (`COLLAPSE`), and those keys in `lib/storage.ts`. The rail calls `useRail("<id>")` and hands what it
+   returns to `ui/FilterRail`; tsc names each table still missing the new id.
 
 What holds it: `moduleRegistry.contract.test.ts` (every id defined once, from its own folder, lazily),
 `moduleStores.contract.test.tsx` (no store reads another module), the `local/layers` lint rule (imports only go