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/app.go

@@ -32,11 +32,12 @@ type view struct {
 
 // App is the top-level arr-tui application.
 type App struct {
-	app    *tview.Application
-	theme  theme         // the colors in use
-	trakt  *trakt.Client // trending, most watched and anticipated lists, casts
-	people *peopleCache  // casts loaded from Trakt
-	disk   *diskCache    // posters and casts kept between runs
+	cellW, cellH int // size of a terminal cell in pixels, 0 if unknown
+	app          *tview.Application
+	theme        theme         // the colors in use
+	trakt        *trakt.Client // trending, most watched and anticipated lists, casts
+	people       *peopleCache  // casts loaded from Trakt
+	disk         *diskCache    // posters and casts kept between runs
 	// Called (on the UI goroutine) a moment after a release was grabbed.
 	afterGrab []func()
 	// The Movies and Series views, to refresh their Trakt lists.
@@ -218,6 +219,7 @@ func New(cfg *config.Config) *App {
 		s.HideCursor()
 		a.screen = s
 		a.cellAspect = cellAspectOf(s)
+		a.cellW, a.cellH = cellPixelsOf(s)
 		a.noteWindow(s)
 		return false
 	})
@@ -240,6 +242,7 @@ func New(cfg *config.Config) *App {
 // helps to find out why a poster is drawn wrongly in some terminal.
 func (a *App) newGraphics() *kittyGraphics {
 	k := newKittyGraphics(a.writeTerminal)
+	k.cellPixels = func() (int, int) { return a.cellW, a.cellH }
 	switch v := os.Getenv("ARR_TUI_DEBUG_GFX"); {
 	case v == "":
 	case strings.Contains(v, "/"):
@@ -298,6 +301,17 @@ func (a *App) writeTerminal(b []byte) error {
 
 // cellAspectOf measures the height of a terminal cell divided by its width in
 // pixels; 2 when the terminal does not report pixel sizes.
+// cellPixelsOf returns the size of a terminal cell in pixels, 0, 0 when the
+// terminal does not say.
+func cellPixelsOf(s tcell.Screen) (w, h int) {
+	if tty, ok := s.Tty(); ok {
+		if ws, err := tty.WindowSize(); err == nil && ws.Width > 0 && ws.Height > 0 {
+			return ws.PixelWidth / ws.Width, ws.PixelHeight / ws.Height
+		}
+	}
+	return 0, 0
+}
+
 func cellAspectOf(s tcell.Screen) float64 {
 	if tty, ok := s.Tty(); ok {
 		if ws, err := tty.WindowSize(); err == nil {