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

docs/gates.md

@@ -61,7 +61,8 @@ them itself (`no-undef`, `no-redeclare`, `no-dupe-keys`, …), nor the formattin
 | src tests · ui tests · lib tests | `local/literal-a11y-text`, `local/input-primitive`, `local/surface-role`, `local/glyph-link` — off | A test renders throwaway markup (a raw input as a focus target, an untranslated label) that is not product UI. `local/button-primitive` stays on: in a test file (and in a control primitive) it only refuses a raw `<button disabled>`, which drops focus there as anywhere. | — |
 | design-lab | `local/literal-a11y-text` — off | The dev-only design lab (never in the bundle — devOnlyGuard fails the build if it is) holds the owner's working copy, not product copy. | — |
 | e2e | `react-hooks/*`, `react-refresh/*`, `local/*`, `no-console`, `no-restricted-globals`, `no-restricted-properties`, `no-restricted-syntax` — not applied | Playwright specs drive a browser from Node: no React, no product UI, and they print to the console on purpose. They get the base + typed rules + the playwright rules. | — |
-| ts configs | `react-hooks/*`, `react-refresh/*`, `local/*`, `no-console`, `no-restricted-globals`, `no-restricted-properties`, `no-restricted-syntax` — not applied | The build configs (vite, vitest, playwright, reactPlugin, devOnlyGuard) run in Node at build time. | — |
+| ts configs · tool tests | `react-hooks/*`, `react-refresh/*`, `local/*`, `no-console`, `no-restricted-globals`, `no-restricted-properties`, `no-restricted-syntax` — not applied | The build configs (vite, vitest, playwright, reactPlugin, devOnlyGuard, chunkGuard) run in Node at build time. Their unit tests beside them (`tool tests`: `chunkGuard.test.ts`) are linted the same way, plus `vitest/no-focused-tests` and `vitest/no-disabled-tests`, since vitest runs them. | — |
+| src root tests · design-lab | `local/layers` — not applied (inside the rule: its `free` check, which the effective config cannot show) | The app-wide tests at `src/` root exercise a behaviour across modules (a write failure reported the same way in Plex and Files, the playlist editors' failures), so they render module-private components directly; the dev-only design lab shows every component. Neither ships. Every other layer, test or not, is held. | 3 imports of module-private files (`PlaylistEditFailures.dom.test.tsx` → `modules/plex/PlexPlaylistView`; `WriteFailureReporting.dom.test.tsx` → `modules/plex/PlexCollectionEditor`, `modules/files/UploadDialog`); design-lab 0 |
 | js configs | `@typescript-eslint/*`, `react-hooks/*`, `react-refresh/*`, `local/*`, `no-console`, `no-restricted-globals`, `no-restricted-properties`, `no-restricted-syntax`, `no-var`, `prefer-const`, `prefer-rest-params`, `prefer-spread`, `no-array-constructor`, `no-unused-expressions` — not applied | `eslint.config.js` and `postcss.config.js`: plain-JS build config, linted with `js.configs.recommended` only. | — |
 | repo scripts | `scripts/*.mjs` — not in the eslint or prettier lane; `js.configs.recommended` + Prettier via `src/lib/repoScripts.contract.test.ts` | They sit at the repo root, and an ESLint config cannot reach past its own directory. The contract test lints and format-checks them from the repo root, to the same bar as the `js configs` row; no typed rules, as they are in no tsc project. | 0 lint hits; 2 files reformatted (E32 review) |
 | everywhere | ignored paths: `dist`, `node_modules`, `playwright-report`, `test-results`, `e2e/.auth` | Build output, installed packages, test artefacts, a live Playwright session. Generated, not written. | — |