feat(detail): like/dislike buttons on the manga page

Add a tri-state like/dislike toggle beside the bookmark action (signed-in
only): click a reaction to set it, the other to switch, or the active one to
clear — optimistic with rollback, mirroring the bookmark toggle. Seeds from
the new GET /me/reactions/:id in the detail loader (non-critical: any failure
degrades to an unset toggle). Unit tests cover the state machine; e2e drives
set → switch → clear against the real endpoints.

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

frontend/src/routes/manga/[id]/+page.svelte

@@ -20,6 +20,7 @@
     import { session } from '$lib/session.svelte';
     import Chip from '$lib/components/Chip.svelte';
     import MangaCard from '$lib/components/MangaCard.svelte';
+    import ReactionButtons from '$lib/components/ReactionButtons.svelte';
     import Sheet from '$lib/components/Sheet.svelte';
     import AddToCollectionModal from '$lib/components/AddToCollectionModal.svelte';
     import Plus from '@lucide/svelte/icons/plus';
@@ -541,6 +542,7 @@
                     >
                         {mangaBookmark ? '★ Bookmarked' : '☆ Bookmark'}
                     </button>
+                    <ReactionButtons mangaId={manga.id} initial={data.reaction} />
                     <button
                         type="button"
                         class="action"