fix(sort): harden shared sort primitives (R13 S1 review round 2)

- SortControl no longer renders a blank select when an always-sorted view
  (no defaultLabel) is handed a key its options don't offer: it falls back to
  the first option instead of value="" (which also tripped React's controlled
  warning). Robustness now lives in the primitive, not each caller.
- PLEX_SORT_FALLBACK is Object.frozen: it's handed out by reference from
  healSort/healPlexSort and stored as provider state, so freezing turns any
  accidental in-place SortState mutation into a loud error instead of silently
  corrupting the shared fallback.
- Test: SortControl falls back to the first option for an unavailable key.

Gate green: tsc/eslint(0 err)/prettier/knip clean, vitest 106.

frontend/src/lib/plexSort.ts

@@ -12,8 +12,10 @@ function plexSortKeys(scope: string): string[] {
   return [...(scope === "movie" ? MOVIE_SORTS : SHOW_SORTS)];
 }
 
-/** The historical default: `sort=title`, `sort_dir=asc`. */
-export const PLEX_SORT_FALLBACK: SortState = { key: "title", dir: "asc" };
+/** The historical default: `sort=title`, `sort_dir=asc`. Frozen: it is handed out by reference (from
+ * healSort/healPlexSort and as the provider's default state), so an accidental in-place mutation of a
+ * SortState would otherwise corrupt the shared fallback for every caller. */
+export const PLEX_SORT_FALLBACK: SortState = Object.freeze({ key: "title", dir: "asc" as const });
 
 /** Labelled, alpha-ordered options for the SortControl. `label` resolves each key (i18n
  * `plex.sort.<key>`). */