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  hardcoded English titles superseded by i18n).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

@@ -10,7 +10,7 @@ export const LANGUAGES = [
 ] as const;
 export type LangCode = (typeof LANGUAGES)[number]["code"];
 const SUPPORTED = LANGUAGES.map((l) => l.code) as readonly string[];
-export const LANG_KEY = LS.lang;
+const LANG_KEY = LS.lang;
 
 // Auto-load every locale file (locales/<lang>/<area>.json) and merge each <area> under one
 // `translation` namespace, so components use t("area.key") and adding an area needs no wiring.
@@ -33,7 +33,7 @@ export function isSupported(code: string | null | undefined): code is LangCode {
 
 // Initial language before the server preference is known: stored choice, else the browser
 // language, else English. (After login, App adopts the server-persisted preference.)
-export function detectInitialLang(): LangCode {
+function detectInitialLang(): LangCode {
   const stored = localStorage.getItem(LANG_KEY);
   if (isSupported(stored)) return stored;
   const nav = (navigator.language || "en").slice(0, 2).toLowerCase();