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/db/Schema.go

@@ -50,10 +50,12 @@ type SolfeggioFrequency struct {
 	Value int
 }
 
-type AudioFile struct {
-	ID       int `gorm:"primaryKey"`
-	Name     string
-	Path     string
-	Selected bool
-	Order    int
+type AudioTrack struct {
+	ID                int `gorm:"primaryKey"`
+	Filename          string
+	Name              string
+	AssignedFrequency int
+	MD5               string
+	Selected          bool
+	Order             int
 }