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/download/download_test.go

@@ -0,0 +1,180 @@
+package download_test
+
+import (
+	"crypto/md5"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+	"rhemawavetech.com/rhemawave/internal/db"
+	"rhemawavetech.com/rhemawave/internal/download"
+)
+
+func md5Hex(data []byte) string {
+	return fmt.Sprintf("%x", md5.Sum(data))
+}
+
+// TestDownloadFileHashChanged verifies that when the server manifest reports a
+// new MD5 for an existing local file, the old file is removed, the new version
+// is downloaded, and the database record is updated with the new hash.
+func TestDownloadFileHashChanged(t *testing.T) {
+	audioDir := t.TempDir()
+	dbDir := t.TempDir()
+
+	gdb, err := gorm.Open(sqlite.Open(filepath.Join(dbDir, "test.db")), &gorm.Config{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	sqlDB, err := gdb.DB()
+	if err != nil {
+		t.Fatal(err)
+	}
+	sqlDB.SetMaxOpenConns(1)
+	sqlDB.SetMaxIdleConns(1)
+
+	if err := gdb.AutoMigrate(&db.AudioTrack{}); err != nil {
+		t.Fatal(err)
+	}
+
+	oldContent := []byte("this is the OLD version of the audio file")
+	oldMD5 := md5Hex(oldContent)
+	newContent := []byte("this is the NEW version of the audio file")
+	newMD5 := md5Hex(newContent)
+
+	track := db.AudioTrack{
+		Filename:          "174Hz.wav",
+		Name:              "174 Hz",
+		AssignedFrequency: 174,
+		MD5:               oldMD5,
+		Selected:          true,
+		Order:             2,
+	}
+	if err := gdb.Create(&track).Error; err != nil {
+		t.Fatal(err)
+	}
+
+	oldPath := filepath.Join(audioDir, "174Hz.wav")
+	if err := os.WriteFile(oldPath, oldContent, 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/checksums.md5":
+			w.WriteHeader(http.StatusOK)
+			fmt.Fprintf(w, "%s  174Hz.wav\n", newMD5)
+		case "/174Hz.wav":
+			w.WriteHeader(http.StatusOK)
+			w.Write(newContent)
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer server.Close()
+
+	progress := make(chan download.DownloadStatus, 100)
+	done := make(chan struct{})
+	go func() {
+		download.DownloadAudioFiles(progress, gdb, audioDir, server.URL+"/")
+		close(done)
+	}()
+	for range progress {
+	}
+	<-done
+
+	newPath := filepath.Join(audioDir, "174Hz.wav")
+	gotContent, err := os.ReadFile(newPath)
+	if err != nil {
+		t.Fatalf("reading new file: %v", err)
+	}
+	if string(gotContent) != string(newContent) {
+		t.Errorf("new file content = %q, want %q", string(gotContent), string(newContent))
+	}
+
+	var updated db.AudioTrack
+	if err := gdb.First(&updated, track.ID).Error; err != nil {
+		t.Fatalf("fetching updated track: %v", err)
+	}
+	if updated.MD5 != newMD5 {
+		t.Errorf("DB MD5 = %q, want %q", updated.MD5, newMD5)
+	}
+}
+
+// TestDownloadOrphanRemoval verifies that files and DB records not in the
+// manifest are removed.
+func TestDownloadOrphanRemoval(t *testing.T) {
+	audioDir := t.TempDir()
+	dbDir := t.TempDir()
+
+	gdb, err := gorm.Open(sqlite.Open(filepath.Join(dbDir, "test.db")), &gorm.Config{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	sqlDB, err := gdb.DB()
+	if err != nil {
+		t.Fatal(err)
+	}
+	sqlDB.SetMaxOpenConns(1)
+	sqlDB.SetMaxIdleConns(1)
+
+	if err := gdb.AutoMigrate(&db.AudioTrack{}); err != nil {
+		t.Fatal(err)
+	}
+
+	orphanPath := filepath.Join(audioDir, "orphan.wav")
+	if err := os.WriteFile(orphanPath, []byte("orphan"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	gdb.Create(&db.AudioTrack{Filename: "ghost.wav", Name: "Ghost Track", Order: 99})
+
+	keptContent := []byte("kept file")
+	keptMD5 := md5Hex(keptContent)
+	keptPath := filepath.Join(audioDir, "kept.wav")
+	if err := os.WriteFile(keptPath, keptContent, 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/checksums.md5":
+			w.WriteHeader(http.StatusOK)
+			fmt.Fprintf(w, "%s  kept.wav\n", keptMD5)
+		case "/kept.wav":
+			w.WriteHeader(http.StatusOK)
+			w.Write(keptContent)
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer server.Close()
+
+	progress := make(chan download.DownloadStatus, 100)
+	done := make(chan struct{})
+	go func() {
+		download.DownloadAudioFiles(progress, gdb, audioDir, server.URL+"/")
+		close(done)
+	}()
+	for range progress {
+	}
+	<-done
+
+	if _, err := os.Stat(orphanPath); !os.IsNotExist(err) {
+		t.Errorf("orphan file %s should have been removed", orphanPath)
+	}
+
+	var count int64
+	gdb.Model(&db.AudioTrack{}).Where("filename = ?", "ghost.wav").Count(&count)
+	if count != 0 {
+		t.Errorf("orphan DB record ghost.wav should have been deleted, count=%d", count)
+	}
+
+	if _, err := os.Stat(keptPath); os.IsNotExist(err) {
+		t.Errorf("kept file %s should still exist after download", keptPath)
+	}
+}