fix(mangas): address review on sort feature — alias, index, validation, a11y

Backend
- sort=recent stays a back-compat alias for created (parse_sort); invalid
  sort/order now return the structured 422 envelope instead of plain-text 400
- per-field default direction on omitted order (dates desc, text asc), matching
  the frontend so a bare ?sort=<field> reads the same in UI and API
- migration 0033: index mangas(updated_at DESC, id) backing the default sort
  and its id tie-break; NULLS LAST now applied only to the nullable author key
- tests: recent alias, per-field default (title+author), invalid-value 422,
  and a tie-break test that pins ordering by ascending id (mutation-verified)

Frontend
- pure sort helpers extracted to $lib/mangaSort with unit tests; coerceSort
  honors the recent alias so pasted/legacy URLs resolve to the same field
- SegmentedControl: roving tabindex + arrow/Home/End keyboard nav, anchored on
  focus so rapid keypresses don't stick
- UX: visible "Direction" labels (desktop + mobile), mobile sort-sheet section
  headings and a "Done" button; README sort/order contract updated

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/e2e/mobile-list-search.spec.ts

@@ -185,5 +185,10 @@ test.describe('mobile catalog chrome', () => {
         await page.getByTestId('sort-order-mobile-desc').click();
         await expect(page).toHaveURL(/order=desc/);
         expect(lastOrderParam).toBe('desc');
+
+        // The Done button gives an explicit dismissal affordance once the
+        // field + direction are set.
+        await page.getByTestId('sort-done').click();
+        await expect(page.getByTestId('sort-sheet')).toBeHidden();
     });
 });