fix(anilist): repair SaveMediaListEntry mutation and surface update failures

The AniListUpdateEntry mutation was mangled in 54c109a when the
standardized media field block was pasted in without the media { }
wrapper: media-level fields (idMal, title, ...) sat directly on
SaveMediaListEntry (which returns MediaList), the MediaList fields
(status, startedAt, ..., user) ended up at the Mutation root, and a
stray closing brace made the document a guaranteed 400. Because the
response status was discarded, the frontend received a zeroed
AniListGetSingleAnime and blanked the anime page after every submit,
making AniList appear logged out (and the change was never saved).

- Restore the mutation to match the tested bruno request: id/mediaId/
  userId, standard media block inside media { }, MediaList fields
  inside the selection, balanced braces.
- AniListUpdateEntry and AniListDeleteEntry now return an error on
  403/non-200/unparseable responses (mirroring
  GetAniListUserWatchingList) instead of silently returning zero
  values.
- Anime.svelte guards against replacing the page data with an empty
  response and raises the API error modal instead.

Bump productVersion to 1.6.1.

frontend/src/helperComponents/Anime.svelte

@@ -4,6 +4,7 @@
     aniListLoggedIn,
     malAnime,
     malLoggedIn,
+    setApiError,
     simklAnime,
     simklLoggedIn,
     watchlistNeedsRefresh,
@@ -207,6 +208,15 @@
           completedAt: convertDateToAniList(completedAtDate),
         };
         await AniListUpdateEntry(body).then((value: AniListGetSingleAnime) => {
+          if (!value.data?.MediaList || value.data.MediaList.mediaId === 0) {
+            setApiError(
+              "anilist",
+              "AniList update failed: no saved entry was returned",
+              undefined,
+              true,
+            );
+            return;
+          }
           value.data.MediaList.media.tags =
             currentAniListAnime.data.MediaList.media.tags;
           value.data.MediaList.media.genres =