feat(mangas): per-field sort options + direction toggle on the catalog (0.88.0)

Sort the manga catalog by created / updated / title / author with an
independent asc/desc direction control. An omitted `order` defaults per field
(dates newest-first, text A→Z), matching the UI, so a bare `?sort=<field>`
means the same thing in the browser and over the API; `sort=recent` remains a
back-compat alias for `created`.

- Backend: SortField/SortOrder parsed with validation (structured 422 on bad
  input), per-field default_order, NULLS LAST only on the nullable author key,
  and migration 0033 indexing mangas(updated_at DESC, id) to back the default
  sort and its id tie-break.
- Frontend: catalog sort field + direction (SegmentedControl) on desktop and a
  mobile bottom sheet; pure helpers in $lib/mangaSort; keyboard-accessible
  direction control; visible Direction labels and a sheet "Done" button.
- Tests: backend integration coverage (defaults, alias, invalid input,
  ascending-id tie-break), frontend unit + e2e.

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

backend/migrations/0033_mangas_updated_at_index.sql

@@ -0,0 +1,16 @@
+-- Back the default catalog ordering. Since 0.88.0 the catalog list defaults to
+-- `ORDER BY updated_at DESC, id` (the no-filter default path), but the only
+-- existing updated_at index on mangas is the PARTIAL `mangas_missing_cover_idx`
+-- (WHERE cover_image_path IS NULL), which Postgres can't use for the unfiltered
+-- ordering. `created_at` and `lower(title)` already have full indexes from
+-- 0001; this gives the new default sort column the same treatment.
+--
+-- `id` is included as a trailing key so the index also covers the stable
+-- `, id` tie-break the listing appends for deterministic pagination.
+--
+-- Not CONCURRENTLY: sqlx::migrate! wraps each migration in a transaction, and
+-- CREATE INDEX CONCURRENTLY can't run inside one. The table is small at our
+-- scale, so a brief lock on an online deploy is acceptable.
+
+CREATE INDEX IF NOT EXISTS mangas_updated_at_idx
+    ON mangas (updated_at DESC, id);