Replace config file with folder-based mapping

Mappings between local playlist folders and Kreativ-Tonies are now
expressed purely through the filesystem instead of a YAML config:

  <root>/<household_id>/<freier_name> [id=<tonie_id>]/*.mp3

- internal/library: discovers tonie folders from this structure
  (Discover/Filter), and scaffolds it automatically from the
  TonieCloud account (Scaffold), leaving already-existing folders
  (matched by tonie id) untouched
- internal/syncer: decoupled from the removed config package via a
  plain Target{TonieID, Folder, Prune} struct
- cmd/toni-sync: new `init --root PATH` command to scaffold the
  library; `sync [FILTER] --root PATH` now discovers targets from the
  folder tree instead of a config file; removed the `config` command
  group entirely
- Unit tests for folder discovery, filtering, and scaffolding

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

internal/library/scaffold_test.go

@@ -0,0 +1,80 @@
+package library
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/arnef/toni-sync/internal/tonieapi"
+)
+
+type fakeScaffoldClient struct {
+	households    []tonieapi.Household
+	toniesByHouse map[string][]tonieapi.CreativeTonie
+}
+
+func (f *fakeScaffoldClient) GetHouseholds() ([]tonieapi.Household, error) {
+	return f.households, nil
+}
+
+func (f *fakeScaffoldClient) GetCreativeTonies(householdID string) ([]tonieapi.CreativeTonie, error) {
+	return f.toniesByHouse[householdID], nil
+}
+
+func TestScaffoldCreatesFolders(t *testing.T) {
+	root := t.TempDir()
+	client := &fakeScaffoldClient{
+		households: []tonieapi.Household{{ID: "h1", Name: "Family"}},
+		toniesByHouse: map[string][]tonieapi.CreativeTonie{
+			"h1": {{ID: "t1", HouseholdID: "h1", Name: "Peppa Wutz"}},
+		},
+	}
+
+	results, err := Scaffold(client, root)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(results) != 1 || !results[0].Created {
+		t.Fatalf("expected 1 created result, got %+v", results)
+	}
+
+	expected := filepath.Join(root, "h1", "Peppa Wutz [id=t1]")
+	if info, err := os.Stat(expected); err != nil || !info.IsDir() {
+		t.Fatalf("expected folder %s to exist: %v", expected, err)
+	}
+}
+
+func TestScaffoldLeavesExistingFolderUntouched(t *testing.T) {
+	root := t.TempDir()
+	// Simulate an existing folder that was renamed locally but keeps the id suffix.
+	existing := filepath.Join(root, "h1", "My Custom Name [id=t1]")
+	if err := os.MkdirAll(existing, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	// Put a marker file inside to make sure Scaffold doesn't touch it.
+	marker := filepath.Join(existing, "song.mp3")
+	if err := os.WriteFile(marker, []byte("data"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	client := &fakeScaffoldClient{
+		households: []tonieapi.Household{{ID: "h1", Name: "Family"}},
+		toniesByHouse: map[string][]tonieapi.CreativeTonie{
+			"h1": {{ID: "t1", HouseholdID: "h1", Name: "Peppa Wutz (renamed in cloud)"}},
+		},
+	}
+
+	results, err := Scaffold(client, root)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(results) != 1 || results[0].Created {
+		t.Fatalf("expected existing folder to be reported as not created, got %+v", results)
+	}
+	if results[0].Folder != existing {
+		t.Fatalf("expected folder %s, got %s", existing, results[0].Folder)
+	}
+	if _, err := os.Stat(marker); err != nil {
+		t.Fatalf("expected local file to remain untouched: %v", err)
+	}
+}