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/components/SortControl.dom.test.tsx

@@ -47,6 +47,11 @@ describe("SortControl", () => {
     expect(none.value).toBe("");
   });
 
+  it("falls back to the first option (not a blank select) for an unavailable key without defaultLabel", () => {
+    renderControl({ key: "ghost", dir: "asc" });
+    expect((screen.getByRole("combobox") as HTMLSelectElement).value).toBe("date");
+  });
+
   it("picking a new option fires onChange at that option's defaultDir", () => {
     const { onChange } = renderControl({ key: "title", dir: "asc" });
     fireEvent.change(screen.getByRole("combobox"), { target: { value: "date" } });