feat(mangas): add tag-similarity "Similar" section on manga detail

Add GET /v1/mangas/:id/similar returning the top 5 mangas ranked by
Jaccard tag overlap (shared / union), excluding self, as MangaCard items
in a plain { items } object. Surface them in a hidden-when-empty
"Similar" section on the manga detail page, reusing MangaCard.

Backend: new repo::manga::list_similar with a manga_tags self-join; a
shared cards_from_rows helper (also used by list_cards) that loads
authors+genres concurrently; single-sourced column list via MANGA_COLS +
manga_cols(alias) to replace the fragile SELECT_COLS string-splitting.

Frontend: getSimilarMangas client fn (guards a malformed body), loader
fetch with non-critical .catch fallback, and the catalog grid hoisted to
a shared global .manga-grid (lib/styles/tokens.css), de-duplicating the
per-route copies.

Bump version 0.62.0 -> 0.63.0 (backend + frontend in lockstep).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/styles/tokens.css

@@ -303,3 +303,24 @@ img {
         animation-duration: 0ms !important;
     }
 }
+
+/* Shared grid for manga cover cards — used by the catalog, author and
+   collection pages, and the similar-mangas section. Single-sourced here
+   so the responsive layout can't drift between pages. Four covers per
+   row on phones is a deliberate user preference; `minmax(0, 1fr)` is
+   load-bearing so the grid never overflows the viewport. */
+.manga-grid {
+    list-style: none;
+    padding: 0;
+    margin: 0;
+    display: grid;
+    grid-template-columns: repeat(auto-fill, minmax(140px, 1fr));
+    gap: var(--space-4);
+}
+
+@media (max-width: 640px) {
+    .manga-grid {
+        grid-template-columns: repeat(4, minmax(0, 1fr));
+        gap: var(--space-2);
+    }
+}