feat(frontend): the folders are layers — an import only goes down, a module is reached through its index.ts

`local/layers` (eslint.config.js) resolves every import to its file and refuses one that goes up
(lib/ → components/ui/ → features/ → modules/ → shell/) or that reaches into another module's own
files. A module's index.ts is its public face: the stores, mounted at the root and read app-wide,
which features/ may read too; a page is still opened lazily by import(). Its contract test probes
every edge, and three mutants of the rule (off, no face check, a door for everyone) each turn it red.

Six modules got that face; the shared channel UI (about, tags, tag manager) moved to
features/channel, the filter chips and the saved-views context to features/filters, and the prefs
store's controller type moved next to the store instead of importing it from its own page. The
header's per-module controls and the eagerly loaded rails keep eight inline suppressions, each
naming why: they go when the header gets slots and every rail loads lazily.

`chunkGuard` fails the production build when a lazily imported module lands in a boot chunk (the
entry, the shell, what they import) — e.g. a page re-exported from an index.ts, which makes its
import() a silent no-op. Proved on a build with that mutant; today's build has none.

frontend/src/shell/App.tsx

@@ -21,8 +21,12 @@ import { LS } from "../lib/storage";
 import { usePref } from "../lib/usePref";
 import { useVerifyEntry } from "../lib/useVerifyEntry";
 import { useNavigation, useNavigationActions } from "../features/navigation/navigationContext";
-import { useFeedFilters, useFeedFiltersActions } from "../modules/feed/feedFiltersContext";
-import { useFeedView, useFeedViewActions } from "../modules/feed/feedViewContext";
+import {
+  useFeedFilters,
+  useFeedFiltersActions,
+  useFeedView,
+  useFeedViewActions,
+} from "../modules/feed";
 import { moduleOrder } from "../lib/modules";
 import { PAGE_CONTENT, PAGE_RAIL, preloadPages } from "./moduleRegistry";
 import { lazyWithPreload } from "../lib/lazyWithPreload";