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/modules/settings/ApiTokens.tsx

@@ -18,7 +18,7 @@ import { compareText } from "../../lib/collate";
 import { formatDate } from "../../lib/format";
 import { navItemAttrs } from "../../lib/listNav";
 import { useConfirm } from "../../components/ui/confirmContext";
-import { folderIcon, folderLabel, takesUploads } from "../files/fileFolders";
+import { folderIcon, folderLabel, takesUploads } from "../files";
 import { type ActionMenuItem } from "../../components/ui/ActionMenu";
 import { Badge } from "../../components/ui/Badge";
 import { Button } from "../../components/ui/Button";

frontend/src/modules/settings/SettingsPanel.tsx

@@ -20,7 +20,7 @@ import { DownloadFormats } from "./ProfileEditor";
 import DiagnosticsSettings from "./DiagnosticsSettings";
 import ApiTokens from "./ApiTokens";
 import { type SettingsTabId } from "../../lib/settingsTab";
-import { SCHEMES, type Scheme, type ThemePrefs } from "../../lib/theme";
+import { SCHEMES, type Scheme } from "../../lib/theme";
 import { api, HttpError, type Me } from "../../lib/api";
 import { displayName, roleKey } from "../../lib/api/account";
 import { notifyActionError } from "../../lib/actionErrors";
@@ -40,7 +40,7 @@ import {
   START_ON_HOME_PREF,
 } from "../../features/navigation/navigationInit";
 import { usePref } from "../../lib/usePref";
-import { usePrefs } from "../../features/prefs/prefsContext";
+import { usePrefs, type PrefsController } from "../../features/prefs/prefsContext";
 import Tabs from "../../components/ui/Tabs";
 import { useTabPanel } from "../../components/ui/tabPanel";
 import { useMe } from "../../lib/useMe";
@@ -49,25 +49,6 @@ import { Slider } from "../../components/ui/NativeControls";
 import { Input, Textarea } from "../../components/ui/Input";
 import { SURFACE } from "../../components/ui/surfaces";
 
-// The Settings page edits server-persisted preferences. Each change applies locally for instant
-// preview AND auto-saves (debounced) — no draft, no Save button; a small indicator flashes the
-// outcome. PrefsProvider owns the state + the save; this controller
-// is how the panel reads/writes it.
-export type PrefsSaveState = SaveState;
-export interface PrefsController {
-  theme: ThemePrefs;
-  setTheme: (t: ThemePrefs) => void;
-  perf: boolean;
-  setPerf: (v: boolean) => void;
-  hints: boolean;
-  setHints: (v: boolean) => void;
-  railLabels: boolean;
-  setRailLabels: (v: boolean) => void;
-  notif: NotifSettings;
-  setNotif: (n: NotifSettings) => void;
-  saveState: PrefsSaveState;
-}
-
 type TabId = SettingsTabId;
 const TABS: { id: TabId; icon: typeof Monitor }[] = [
   { id: "appearance", icon: Monitor },
@@ -138,7 +119,7 @@ export default function SettingsPanel() {
 
 // A small transient indicator of the auto-save outcome. Renders nothing while idle; it sits at the
 // bottom of the settings card and fades after a success (the state resets to "idle" in App).
-function SaveIndicator({ state }: { state: PrefsSaveState }) {
+function SaveIndicator({ state }: { state: SaveState }) {
   const { t } = useTranslation();
   if (state === "idle") return null;
   return (