feat(theme): --scale for retina gallery captures

deviceScaleFactor pass-through to chromedp EmulateViewport; --scale 2
produces 2880x1800 PNGs for hidpi store galleries.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

cmd/ninja/cmd/theme.go

@@ -26,6 +26,7 @@ func newThemeScreenshotCmd() *cobra.Command {
 	var pages, screenshotsDir, modes string
 	var mobile bool
 	var width, height int
+	var scale float64
 	cmd := &cobra.Command{
 		Use:   "screenshot",
 		Short: "Render this theme's showcase page and write preview.png into the repo",
@@ -68,6 +69,7 @@ the rest of the CLI.`,
 				Height:       height,
 				WaitSelector: waitSelector,
 				Timeout:      45 * time.Second,
+				Scale:        scale,
 			})
 			if err != nil {
 				return err
@@ -86,6 +88,7 @@ the rest of the CLI.`,
 					Height:       844,
 					WaitSelector: waitSelector,
 					Timeout:      45 * time.Second,
+					Scale:        scale,
 				})
 				if err != nil {
 					return err
@@ -124,6 +127,7 @@ the rest of the CLI.`,
 							WaitSelector: waitSelector,
 							Timeout:      45 * time.Second,
 							Mode:         mode,
+							Scale:        scale,
 						})
 						if err != nil {
 							return err
@@ -155,6 +159,7 @@ the rest of the CLI.`,
 	cmd.Flags().IntVar(&height, "height", 900, "Desktop viewport height")
 	cmd.Flags().StringVar(&pages, "pages", "", "Comma-separated page slugs to capture into --screenshots-dir (e.g. /,/blog,/about)")
 	cmd.Flags().StringVar(&screenshotsDir, "screenshots-dir", "screenshots", "Directory for --pages captures (packed into the .bnp)")
+	cmd.Flags().Float64Var(&scale, "scale", 1, "Device scale factor (2 = retina PNGs at Width*2 x Height*2)")
 	cmd.Flags().StringVar(&modes, "modes", "", `Comma-separated color modes for --pages captures: "light", "dark" or "light,dark" (sets the bn-theme cookie per capture; empty = site default, unsuffixed filenames)`)
 	return cmd
 }

internal/shot/shot.go

@@ -25,6 +25,7 @@ type Options struct {
 	WaitSelector string        // CSS selector to wait for before capturing (e.g. "section")
 	Timeout      time.Duration // overall navigation+capture timeout
 	Mode         string        // "" (site default), "light" or "dark" — sets the bn-theme cookie + emulated prefers-color-scheme before navigation
+	Scale        float64       // device scale factor; 0 = 1.0. 2 captures retina PNGs (Width*2 × Height*2 px)
 }
 
 // PreviewURL composes the gallery-page URL with the render-only override.
@@ -68,8 +69,12 @@ func Capture(ctx context.Context, opts Options) ([]byte, error) {
 	defer cancelTimeout()
 
 	var buf []byte
+	viewport := []chromedp.EmulateViewportOption(nil)
+	if opts.Scale > 0 {
+		viewport = append(viewport, chromedp.EmulateScale(opts.Scale))
+	}
 	tasks := chromedp.Tasks{
-		chromedp.EmulateViewport(int64(opts.Width), int64(opts.Height)),
+		chromedp.EmulateViewport(int64(opts.Width), int64(opts.Height), viewport...),
 	}
 	if opts.Mode == "light" || opts.Mode == "dark" {
 		u, err := url.Parse(opts.URL)