Replace AudioFile with AudioTrack, fix download+orphan+MD5 sync, always play original audio fallback

AudioTrack table replaces AudioFile (Filename, Name, AssignedFrequency,
MD5, Selected, Order) — fixes relative-path bug in scrollerPage.go that
caused "file not found" for single-track selection.

Download goroutine now:
  - Accepts *gorm.DB and updates AudioTrack.MD5 after each download.
  - Treats checksums.md5 as source of truth: removes local files and DB
    records not in the manifest (orphans).
  - Strips "./" prefix from manifest filenames (GNU md5sum format).

AudioOptions.go seeds 14 AudioTrack records matching server checksums.md5.
List background quad width matches the widest item (+ padding), centered.

scrollerPage.go switchAudioForFrequency falls back to original/selected
audio when a frequency has no matching track or file.  playOriginalAudio
is a no-op on repeat calls to avoid 5-second audio restart loop.

Fixes progress display: passes Max=0 during downloads so Home page shows
"x / X Files Downloaded" text instead of bogus "100.0%" percentage.

Downstream audio switching (DynamicAudio) verifies an AudioTrack exists
for the frequency before assigning it to a stream — frequencies without
a track (e.g. 528 Hz) gracefully use original audio instead.

internal/application/application.go

@@ -195,7 +195,7 @@ func Run() {
 		}
 	}
 
-	go download.DownloadAudioFiles(app.PageRouter.Context.MessageChannel, audioDir, "https://files.radiantwavetech.com/audio/")
+	go download.DownloadAudioFiles(app.PageRouter.Context.MessageChannel, app.DB, audioDir, "https://files.radiantwavetech.com/audio/")
 
 	app.runEventLoop()
 }