Gate READY on download completion; move audio to ~/.radiantwave/audio

State machine now uses DOWNLOADING_ASSETS before READY. Downloads closing
the DownloadDone channel signals checkState() to proceed. Download failure
sets downloadFailed flag which prevents READY. Audio files moved from
~/.local/share/rhemawave/audio to ~/.radiantwave/audio/.

Regression tests in application_test.go (linux build-tagged due to
network package D-Bus init). Version 0.6.

internal/page/router.go

@@ -25,6 +25,7 @@ type PageCtx struct {
 	DbPtr          *gorm.DB
 	W, H           int32
 	MessageChannel chan download.DownloadStatus
+	DownloadDone   chan struct{} // closed when downloads finish (success or failure)
 	NavigateTo     func(pageType)
 	Msg            string // Message to be displayed by MessagePage
 	Inst           string // Instructions to be displayed on MessagePage