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/Sidebar.tsx

@@ -39,29 +39,11 @@ import {
 import { shareUrl } from "../lib/urlState";
 import { notify } from "../lib/notifications";
 import { useDebounced } from "../lib/useDebounced";
-import { Switch } from "./ui/form";
 import TagManager from "./TagManager";
 import SavedViewsWidget from "./SavedViewsWidget";
 import CollapsedFilterRail from "./CollapsedFilterRail";
 
 // Filter ids; display labels resolved at render time via t("sidebar.sort.<id>") etc.
-const SORT_IDS = [
-  "newest",
-  "oldest",
-  "views",
-  "duration_desc",
-  "duration_asc",
-  "title",
-  "subscribers",
-  "priority",
-  "shuffle",
-];
-
-const SHOW_IDS = ["unwatched", "in_progress", "all", "watched", "hidden"];
-
-// Fresh shuffle token for the "surprise me" sort.
-const rollSeed = () => Math.floor(Math.random() * 1_000_000_000);
-
 const DATE_PRESETS: { days: number; key: string }[] = [
   { days: 1, key: "24h" },
   { days: 7, key: "1week" },
@@ -697,19 +679,3 @@ function WidgetCard({
   );
 }
 
-function Toggle({
-  label,
-  checked,
-  onChange,
-}: {
-  label: string;
-  checked: boolean;
-  onChange: (v: boolean) => void;
-}) {
-  return (
-    <label className="flex items-center justify-between py-1 cursor-pointer text-sm">
-      <span>{label}</span>
-      <Switch label={label} checked={checked} onChange={onChange} />
-    </label>
-  );
-}