chore: Phase 1 hygiene — drop unused imports/exports/types (behavior-neutral)

Machine-baseline harvest (ruff + knip), all tsc/parse-green, no runtime change:
- backend: remove 3 unused imports (channels/playlists/youtube) via ruff; drop the
  unused `job` binding in unshare_download (keep the _own_job ownership guard call).
- frontend: remove `export` from 23 internally-used-only symbols (knip "unused
  exports") to shrink the public surface; delete 2 genuinely-dead declarations
  (PlexBrowseResult — leftover from the removed /browse route; WIDGET_TITLES —
  hardcoded English titles superseded by i18n).

Held back for a decision (unused here = possibly-unwired, NOT dead — flagged, not
removed): e2ee.lock()/clearDevice() (security primitives never wired to logout / a
"forget device" feature) and loadDefaultViewFilters (App reimplements it inline — a
DRY issue). See siftlode-ops/CODE-HYGIENE.md.

frontend/src/lib/notifications.ts

@@ -6,7 +6,7 @@ import { LS, readAccount, readAccountMerged, writeAccount } from "./storage";
 
 export type NotifLevel = "info" | "success" | "warning" | "error" | "fatal";
 
-export interface NotifAction {
+interface NotifAction {
   label: string;
   onClick: () => void;
 }
@@ -34,7 +34,7 @@ export type ChannelSubscribedMeta = {
 };
 // Admin nudge that pending access requests are waiting — carries a durable inbox link to the
 // Users page (where Access requests live). No payload; the panel provides the navigation.
-export type AccessRequestsMeta = {
+type AccessRequestsMeta = {
   kind: "access-requests";
 };
 export type NotifMeta =
@@ -224,7 +224,7 @@ export function notify(input: NotifyInput): number {
 }
 
 /** Back-compat: a simple info toast with an optional inline action (e.g. Undo). */
-export function toast(message: string, action?: NotifAction): number {
+function toast(message: string, action?: NotifAction): number {
   return notify({ message, action });
 }