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/PlexBrowse.tsx

@@ -54,7 +54,6 @@ type Sub =
 
 type Props = {
   q: string;
-  onClearSearch: () => void;
   scope: string; // movie | show | both (unified cross-library scope)
   setScope: (v: string) => void;
   show: string;
@@ -70,7 +69,6 @@ const PAGE = 40;
 
 export default function PlexBrowse({
   q,
-  onClearSearch,
   scope,
   setScope,
   show,