Skip empty tonie folders instead of pruning all chapters

Previously, an accidentally empty tonie folder combined with pruning
(the default) would delete every existing chapter on the tonie. Now:

- syncer.BuildPlan detects an empty local folder and returns a
  Plan{Skipped: true, SkipReason: ...} instead of computing a diff
- syncer.ApplyPlan is a no-op for a skipped plan (defense in depth)
- Plan.NeedsChanges() reports false for skipped plans
- cmd/toni-sync sync prints "Skipped: <reason>" for such folders and
  moves on, regardless of --no-prune
- Added a test covering the skip behavior end-to-end (BuildPlan +
  ApplyPlan no-op)
- README documents the safety behavior

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

README.md

@@ -132,6 +132,10 @@ toni-sync sync --root ~/Musik/tonies --dry-run
 existieren (abschaltbar via `--no-prune`), und sortiert die Kapitel passend
 zur lokalen Reihenfolge (Playlist-Datei falls vorhanden, sonst alphabetisch).
 
+**Leere Tonie-Ordner werden übersprungen** - so verhindert toni-sync, dass
+ein versehentlich leerer Ordner (z. B. noch nicht befüllt) beim Sync alle
+vorhandenen Kapitel auf dem Tonie löscht.
+
 ### Household-/Tonie-IDs nachschlagen
 
 Falls du die Struktur lieber manuell pflegen willst:

cmd/toni-sync/sync.go

@@ -77,6 +77,11 @@ func syncOne(client *tonieapi.Client, t library.Tonie, prune, dryRun bool) error
 		return err
 	}
 
+	if plan.Skipped {
+		fmt.Printf("  Skipped: %s\n", plan.SkipReason)
+		return nil
+	}
+
 	if len(plan.ToUpload) > 0 {
 		fmt.Printf("  Upload (%d):\n", len(plan.ToUpload))
 		for _, track := range plan.ToUpload {

internal/syncer/syncer.go

@@ -105,10 +105,18 @@ type Plan struct {
 	ToUpload         []Track
 	ToRemove         []tonieapi.Chapter
 	FinalOrderTitles []string
+
+	// Skipped is true when the plan intentionally does nothing, e.g. because
+	// the local folder is empty. SkipReason explains why.
+	Skipped    bool
+	SkipReason string
 }
 
 // NeedsChanges reports whether applying the plan would change anything on the tonie.
 func (p *Plan) NeedsChanges() bool {
+	if p.Skipped {
+		return false
+	}
 	if len(p.ToUpload) > 0 || len(p.ToRemove) > 0 {
 		return true
 	}
@@ -135,6 +143,11 @@ func (p *Plan) NeedsChanges() bool {
 }
 
 // BuildPlan fetches the current state of the tonie and computes the diff against the local folder.
+//
+// If the local folder contains no audio tracks, the plan is marked as
+// Skipped instead of computing a diff - otherwise an accidentally empty
+// folder (e.g. not yet filled, or a transient sync issue) would delete all
+// chapters on the tonie when pruning is enabled.
 func BuildPlan(client TonieClient, target Target) (*Plan, error) {
 	tonie, err := client.GetCreativeTonie(target.TonieID)
 	if err != nil {
@@ -146,6 +159,15 @@ func BuildPlan(client TonieClient, target Target) (*Plan, error) {
 		return nil, err
 	}
 
+	if len(tracks) == 0 {
+		return &Plan{
+			Tonie:       *tonie,
+			LocalTracks: tracks,
+			Skipped:     true,
+			SkipReason:  "folder is empty - skipping to avoid deleting all chapters on the tonie",
+		}, nil
+	}
+
 	localTitles := make(map[string]bool, len(tracks))
 	orderedTitles := make([]string, 0, len(tracks))
 	for _, t := range tracks {
@@ -191,7 +213,12 @@ func BuildPlan(client TonieClient, target Target) (*Plan, error) {
 }
 
 // ApplyPlan uploads missing tracks, then reorders/prunes chapters to match the local folder.
+// It is a no-op if the plan is Skipped (see BuildPlan).
 func ApplyPlan(client TonieClient, target Target, plan *Plan) error {
+	if plan.Skipped {
+		return nil
+	}
+
 	for _, track := range plan.ToUpload {
 		if err := client.UploadFileToTonie(plan.Tonie, track.Path, track.Title); err != nil {
 			return fmt.Errorf("uploading %s: %w", track.Path, err)

internal/syncer/syncer_test.go

@@ -129,6 +129,32 @@ func TestBuildPlanNoPruneKeepsStaleChapters(t *testing.T) {
 	}
 }
 
+func TestBuildPlanSkipsEmptyFolder(t *testing.T) {
+	dir := t.TempDir() // no audio files
+	tonie := makeTonie([]tonieapi.Chapter{{ID: "c1", Title: "existing chapter"}})
+	client := &fakeClient{tonieSequence: []tonieapi.CreativeTonie{tonie}}
+
+	m := Target{TonieID: "t1", Folder: dir, Prune: true}
+	plan, err := BuildPlan(client, m)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !plan.Skipped {
+		t.Fatal("expected plan to be skipped for empty folder")
+	}
+	if plan.NeedsChanges() {
+		t.Fatal("expected NeedsChanges to be false for a skipped plan")
+	}
+
+	if err := ApplyPlan(client, m, plan); err != nil {
+		t.Fatal(err)
+	}
+	if len(client.sortedTitles) != 0 || len(client.uploadedTitles) != 0 {
+		t.Fatalf("expected ApplyPlan to be a no-op for a skipped plan, got uploads=%v sorted=%v", client.uploadedTitles, client.sortedTitles)
+	}
+}
+
 func TestApplyPlanUploadsAndSkipsReorderWhenAlreadyMatching(t *testing.T) {
 	dir := setupFolder(t)
 	before := makeTonie([]tonieapi.Chapter{{ID: "c1", Title: "01 - First"}})