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/lib/api/reactions.ts

@@ -0,0 +1,42 @@
+import { ApiError, request } from './client';
+
+/** A private per-user taste signal on a manga. */
+export type Reaction = 'like' | 'dislike';
+
+export type MangaReaction = {
+    manga_id: string;
+    reaction: Reaction | null;
+};
+
+/** PUT /v1/mangas/:id/reaction — set (or switch) the user's reaction. */
+export async function setReaction(mangaId: string, reaction: Reaction): Promise<MangaReaction> {
+    return request<MangaReaction>(`/v1/mangas/${encodeURIComponent(mangaId)}/reaction`, {
+        method: 'PUT',
+        headers: { 'content-type': 'application/json' },
+        body: JSON.stringify({ reaction })
+    });
+}
+
+/** DELETE /v1/mangas/:id/reaction — clear the user's reaction (idempotent). */
+export async function clearReaction(mangaId: string): Promise<void> {
+    await request<void>(`/v1/mangas/${encodeURIComponent(mangaId)}/reaction`, {
+        method: 'DELETE'
+    });
+}
+
+/**
+ * Returns the user's reaction for a manga, or `null` when they haven't
+ * reacted (or aren't signed in). Used by the detail page to seed the
+ * like/dislike toggle.
+ */
+export async function getMyReactionForManga(mangaId: string): Promise<Reaction | null> {
+    try {
+        const r = await request<MangaReaction>(
+            `/v1/me/reactions/${encodeURIComponent(mangaId)}`
+        );
+        return r.reaction;
+    } catch (e) {
+        if (e instanceof ApiError && (e.status === 401 || e.status === 404)) return null;
+        throw e;
+    }
+}