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"

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

@@ -2,17 +2,22 @@ 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';
+import { getMyReactionForManga } from '$lib/api/reactions';
 import type { PageLoad } from './$types';
 
 export const ssr = false;
 
 export const load: PageLoad = async ({ params }) => {
-    const [manga, chapters, bookmarks, readProgress, similar] = await Promise.all([
+    const [manga, chapters, bookmarks, readProgress, reaction, similar] = await Promise.all([
         getManga(params.id),
         listChapters(params.id),
         listMyBookmarksOrEmpty(),
         // Null when guest or never-read — page handles both cases.
         getMyReadProgressForManga(params.id),
+        // Null when guest or not reacted — seeds the like/dislike toggle.
+        // Non-critical: any failure degrades to an unset toggle, never a
+        // broken page.
+        getMyReactionForManga(params.id).catch(() => null),
         // 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[])
@@ -22,6 +27,7 @@ export const load: PageLoad = async ({ params }) => {
         chapters: chapters.items,
         bookmarks: bookmarks.items,
         readProgress,
+        reaction,
         similar
     };
 };