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  hardcoded English titles superseded by i18n).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/components/ui/form.tsx

@@ -59,7 +59,7 @@ export function Section({
 
 /** A label that reveals an explanatory caption on hover (dotted underline) when `hint` is set.
  * No-op styling when there's no hint, so it's safe to use unconditionally. */
-export function HintLabel({ hint, children }: { hint?: string; children: ReactNode }) {
+function HintLabel({ hint, children }: { hint?: string; children: ReactNode }) {
   return (
     <Tooltip hint={hint ?? ""}>
       <span