Real-image posters: scale to the box's pixel size

Ghostty shows an image smaller than its placement at its own size, so
library posters (Radarr's poster-500 is 333x500, not 500x750) came out
smaller than the 426x640 remote ones. Images are now scaled (bilinear)
to exactly cols*cellWidth x rows*cellHeight pixels before upload, and
library titles use the original poster for real images. The disk cache
stores posters as decoded (JPEG) instead of the possibly large originals.

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

internal/ui/cover.go

@@ -6,9 +6,9 @@ import (
 	"fmt"
 	"image"
 	"image/color"
-	_ "image/gif"  // poster formats
-	_ "image/jpeg" // (Radarr and Sonarr store the provider's bytes under a .jpg name,
-	_ "image/png"  // whatever format they are in)
+	_ "image/gif" // poster formats (Radarr and Sonarr store the provider's bytes
+	"image/jpeg"  // under a .jpg name, whatever format they are in); jpeg also
+	_ "image/png" // encodes the copies kept on disk
 
 	_ "golang.org/x/image/webp"
 	"net/url"
@@ -563,7 +563,12 @@ func (a *App) loadCover(ref coverRef, done func(*image.RGBA, error)) {
 				img, err = decodeCover(data, maxHeight)
 			}
 			if err == nil {
-				a.disk.writePoster(ref.key, large, data)
+				// Stored as decoded (at most maxHeight tall), not as downloaded:
+				// originals can be megabytes.
+				var shrunk bytes.Buffer
+				if jpeg.Encode(&shrunk, img, &jpeg.Options{Quality: 90}) == nil {
+					a.disk.writePoster(ref.key, large, shrunk.Bytes())
+				}
 			}
 		}
 		a.post(func() {