Series drill-down: season posters

The drill-down reads the series with includeSeasonImages=true (Sonarr
only sends season posters when asked, as TheTVDB links). A selected
season or episode shows its season's poster; without one, or when it
cannot be loaded, the series poster.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ui/library_test.go

@@ -1,6 +1,9 @@
 package ui
 
 import (
+	"context"
+	"net/http"
+	"net/http/httptest"
 	"slices"
 	"strings"
 	"testing"
@@ -308,3 +311,37 @@ func TestCutToWidth(t *testing.T) {
 		}
 	}
 }
+
+func TestSeasonPoster(t *testing.T) {
+	seasonSrv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/broken.jpg" {
+			http.NotFound(w, r)
+			return
+		}
+		w.Write([]byte("SEASON"))
+	}))
+	defer seasonSrv.Close()
+
+	series := coverRef{key: "series", fetch: func(context.Context, bool) ([]byte, error) { return []byte("SERIES"), nil }}
+	v := &seriesView{cover: series, series: sonarr.Series{Seasons: []sonarr.Season{
+		{SeasonNumber: 1, Images: []arr.Image{{CoverType: "poster", RemoteURL: seasonSrv.URL + "/s1.jpg"}}},
+		{SeasonNumber: 2},
+		{SeasonNumber: 3, Images: []arr.Image{{CoverType: "poster", RemoteURL: seasonSrv.URL + "/broken.jpg"}}},
+	}}}
+	fetch := func(season int) string {
+		data, err := v.seasonCover(season).fetch(t.Context(), false)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return string(data)
+	}
+	if got := fetch(1); got != "SEASON" || v.seasonCover(1).key == series.key {
+		t.Errorf("season 1: %q", got)
+	}
+	if v.seasonCover(2).key != series.key {
+		t.Error("a season without a poster shows the series poster")
+	}
+	if got := fetch(3); got != "SERIES" {
+		t.Errorf("a season poster that cannot be loaded falls back to the series poster: %q", got)
+	}
+}