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/routes/manga/[id]/+page.ts

@@ -1,4 +1,4 @@
-import { getManga } from '$lib/api/mangas';
+import { getManga, getSimilarMangas, type MangaCard } from '$lib/api/mangas';
 import { listChapters } from '$lib/api/chapters';
 import { listMyBookmarksOrEmpty } from '$lib/api/bookmarks';
 import { getMyReadProgressForManga } from '$lib/api/read_progress';
@@ -7,17 +7,21 @@ import type { PageLoad } from './$types';
 export const ssr = false;
 
 export const load: PageLoad = async ({ params }) => {
-    const [manga, chapters, bookmarks, readProgress] = await Promise.all([
+    const [manga, chapters, bookmarks, readProgress, similar] = await Promise.all([
         getManga(params.id),
         listChapters(params.id),
         listMyBookmarksOrEmpty(),
         // Null when guest or never-read — page handles both cases.
-        getMyReadProgressForManga(params.id)
+        getMyReadProgressForManga(params.id),
+        // Recommendations are non-critical: a failure here must not break
+        // the detail page, so fall back to an empty list.
+        getSimilarMangas(params.id).catch(() => [] as MangaCard[])
     ]);
     return {
         manga,
         chapters: chapters.items,
         bookmarks: bookmarks.items,
-        readProgress
+        readProgress,
+        similar
     };
 };