Fix double-close panic on DownloadDone channel

When transitioning from DOWNLOADING_ASSETS to READY, checkState()
re-navigates to HOME. The new Home page sees the already-closed
MessageChannel and tries to close DownloadDone again, causing a
panic that restarts the process.

Guard the close with a downloadDone bool field on Home.

internal/page/home.go

@@ -32,6 +32,7 @@ type Home struct {
 	baseFontSize         int
 	isDownloadComplete   bool
 	downloadFailed       bool
+	downloadDone         bool
 }
 
 // Init performs basic page initialization and creation of static assets.
@@ -178,8 +179,11 @@ func (p *Home) Update(dt float32) error {
 				} else {
 					p.setMessage("Status: READY", colors.Green)
 					p.isDownloadComplete = true
+					if !p.downloadDone {
+						p.downloadDone = true
 						close(p.Context.DownloadDone)
 					}
+				}
 				return nil
 			}
 		default: