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  hardcoded English titles superseded by i18n).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/sidebarLayout.ts

@@ -8,15 +8,7 @@ import { LS, readAccount, writeAccount } from "./storage";
 
 export type WidgetId = "savedviews" | "date" | "language" | "topic" | "tags";
 
-export const ALL_WIDGETS: WidgetId[] = ["savedviews", "date", "tags", "language", "topic"];
-
-export const WIDGET_TITLES: Record<WidgetId, string> = {
-  savedviews: "Saved views",
-  date: "Upload date",
-  language: "Language",
-  topic: "Topic",
-  tags: "Tags",
-};
+const ALL_WIDGETS: WidgetId[] = ["savedviews", "date", "tags", "language", "topic"];
 
 export interface SidebarLayout {
   order: WidgetId[];