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/lib/notifications.ts

@@ -223,11 +223,6 @@ export function notify(input: NotifyInput): number {
   return id;
 }
 
-/** Back-compat: a simple info toast with an optional inline action (e.g. Undo). */
-function toast(message: string, action?: NotifAction): number {
-  return notify({ message, action });
-}
-
 /** Close the transient toast surface; the entry stays in the center's history. */
 export function dismiss(id: number): void {
   items = items.map((n) => (n.id === id ? { ...n, dismissed: true } : n));

frontend/src/lib/storage.ts

@@ -129,29 +129,11 @@ export function writeJSON(key: string, value: unknown): void {
 
 // --- reactive persisted string state ----------------------------------------------------
 
-/** A `useState` whose string value is mirrored to localStorage, so it survives a reload (F5).
- * Validation is deferred to the caller (clamp at render) so it can be used before an
- * async-loaded option list is known, keeping hook order stable. Generalizes the per-component
- * "persisted active tab" pattern (Settings, Stats, admin pages, the channel filter…). */
-function usePersistedState(
-  key: string,
-  fallback = "",
-): [string, (value: string) => void] {
-  const [value, setValue] = useState<string>(() => localStorage.getItem(key) ?? fallback);
-  const set = (next: string) => {
-    setValue(next);
-    try {
-      localStorage.setItem(key, next);
-    } catch {
-      /* ignore */
-    }
-  };
-  return [value, set];
-}
-
-/** Like usePersistedState but scoped to the current account (see accountKey), so one account's
- * persisted tab/view position doesn't carry over to another in the same browser. These hooks run
- * in components that only mount once signed in, so the account is known. */
+/** A `useState` whose string value is mirrored to localStorage, scoped to the current account (see
+ * accountKey), so one account's persisted tab/view position doesn't carry over to another in the
+ * same browser. Survives reload (F5); validation is deferred to the caller (clamp at render) so it
+ * can be used before an async-loaded option list is known, keeping hook order stable. These hooks
+ * run in components that only mount once signed in, so the account is known. */
 export function useAccountPersistedState(
   base: string,
   fallback = "",