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  hardcoded English titles superseded by i18n).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

@@ -14,10 +14,10 @@
 
 import { getAccountRaw, LS, setAccountRaw } from "./storage";
 
-export const ONBOARD_ACTIVE = "siftlode.onboarding.active"; // sessionStorage (not in LS, which is localStorage)
+const ONBOARD_ACTIVE = "siftlode.onboarding.active"; // sessionStorage (not in LS, which is localStorage)
 // Per-account (see accountKey): a fresh account should still get the onboarding nudge even if a
 // different account in the same browser dismissed it.
-export const ONBOARD_DISMISSED = LS.onboardingDismissed;
+const ONBOARD_DISMISSED = LS.onboardingDismissed;
 
 export function shouldAutoOpenOnboarding(me: {
   can_read: boolean;