Real-image posters of library titles: use the 500 px copy

Library titles fetched Radarr's/Sonarr's 250×375 poster copy, which
real images show at up to 640 pixels, so movie posters looked soft next
to series posters that came from the provider's full-size original
(Sonarr behind a login). With real images the 500×750 copy is used;
block-character posters keep the small one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

internal/ui/cover_test.go

@@ -165,7 +165,7 @@ func TestLoadCoverCachesAndDedupes(t *testing.T) {
 	var fetches atomic.Int32
 	var buf bytes.Buffer
 	png.Encode(&buf, stripes(10, 20))
-	ref := coverRef{key: "k", fetch: func(context.Context) ([]byte, error) {
+	ref := coverRef{key: "k", fetch: func(context.Context, bool) ([]byte, error) {
 		fetches.Add(1)
 		return buf.Bytes(), nil
 	}}
@@ -186,7 +186,7 @@ func TestLoadCoverRemembersFailures(t *testing.T) {
 	a := newTestApp(t)
 	a.coverCache.failed["bad"] = time.Now()
 	var gotErr error
-	a.loadCover(coverRef{key: "bad", fetch: func(context.Context) ([]byte, error) {
+	a.loadCover(coverRef{key: "bad", fetch: func(context.Context, bool) ([]byte, error) {
 		t.Error("a recent failure must not be retried")
 		return nil, nil
 	}}, func(_ *image.RGBA, err error) { gotErr = err })