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/detail_test.go

@@ -116,7 +116,7 @@ func TestPosterWaitsWhileTheSelectionMoves(t *testing.T) {
 	img := image.NewRGBA(image.Rect(0, 0, 20, 30))
 	a.coverCache.imgs["one"], a.coverCache.imgs["two"] = img, img
 	ref := func(key string) coverRef {
-		return coverRef{key: key, fetch: func(context.Context) ([]byte, error) { return nil, nil }}
+		return coverRef{key: key, fetch: func(context.Context, bool) ([]byte, error) { return nil, nil }}
 	}
 	d := newDetailPane(a)