chore(hygiene): Phase 4 guardrails — noUnusedLocals/Parameters + dead-code sweep

Enable tsconfig noUnusedLocals + noUnusedParameters (permanent guardrail: every tsc/
build now flags re-accumulated dead code). Fixed the 8 violations that surfaced, all
genuine dead code:
- Sidebar: deleted unused SORT_IDS/SHOW_IDS/rollSeed consts + the dead Toggle component
  (+ its now-unused Switch import).
- PlexBrowse: dropped the unused onClearSearch prop (Props + destructure + App call site).
- VideoEditor: dropped the unused map index; notifications: deleted the dead back-compat
  toast(); storage: deleted the unused non-account usePersistedState (+ readJSON import).
- SavedViewsWidget: deleted the dead loadDefaultViewFilters export (App loads the default
  view inline; resolves the last knip unused-export → knip now clean).

Backend: added backend/ruff.toml (ignore E402 — intentional pre-import setup in main.py)
and fixed the 6 E702/E741 style items (_vtt_s_to_ts semicolons, ambiguous `l` loop vars)
so ruff is green. localdev boots; Feed/Plex/sidebar render; 0 console errors.

frontend/src/components/VideoEditor.tsx

@@ -366,7 +366,7 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
               </div>
             )}
             {/* segment tint: dropped = dimmed + hatched */}
-            {segments.map((s, i) => (
+            {segments.map((s) => (
               <div
                 key={s.id}
                 className={clsx(