fix(settings): fail closed on an unreadable lock store and make lock writes reliable

- A lock store that cannot be read is no longer treated as empty: while
  the lock is active every category is withheld (renderer predicates and
  set-based filters alike) until the PIN is entered or the store reads
  again, and writes are refused meanwhile so an empty in-memory store can
  never wipe the persisted locks. The lock set now lives in its own
  ParentalLockLockStore service.
- The M3U group dialog's lock write is awaited and a failed save is
  reported in a snackbar instead of being silently dropped.
- The Electron categories.locked re-stamp clears and re-locks inside one
  transaction, so a failed restamp keeps the previous index.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

libs/shared/interfaces/src/lib/parental-lock.util.ts

@@ -48,6 +48,22 @@ export const PARENTAL_LOCK_XTREAM_CATEGORY_TYPES: readonly ParentalLockXtreamCat
 export const PARENTAL_LOCK_STALKER_CATEGORY_TYPES: readonly ParentalLockStalkerCategoryType[] =
     ['itv', 'vod', 'series', 'radio'];
 
+/**
+ * A "withheld categories" set that withholds EVERY category. Used while the
+ * lock is active but the lock store could not be read: consumers keep their
+ * `size`/`has` checks and simply find nothing they may show.
+ */
+export const ALL_CATEGORIES_WITHHELD: ReadonlySet<string> = Object.freeze(
+    new (class extends Set<string> {
+        override has(): boolean {
+            return true;
+        }
+        override get size(): number {
+            return 1;
+        }
+    })()
+);
+
 export function createEmptyParentalLockPlaylistLocks(): ParentalLockPlaylistLocks {
     return { xtream: [], stalker: [], m3u: [] };
 }