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  hardcoded English titles superseded by i18n).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

@@ -5,7 +5,7 @@ import { createStore } from "./store";
 // server can't carry out an action (a definitive 4xx/5xx response). Distinct from toasts
 // (transient, e.g. connection blips) and from silent handling. Module-level so the non-React
 // api layer can raise it; an <ErrorDialog> mounted at the app root renders the current one.
-export interface AppError {
+interface AppError {
   title: string;
   message: string;
 }