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.go

@@ -0,0 +1,98 @@
+package library
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/arnef/toni-sync/internal/tonieapi"
+)
+
+// ScaffoldClient is the subset of tonieapi.Client needed to scaffold a library folder tree.
+type ScaffoldClient interface {
+	GetHouseholds() ([]tonieapi.Household, error)
+	GetCreativeTonies(householdID string) ([]tonieapi.CreativeTonie, error)
+}
+
+// ScaffoldResult reports what Scaffold did for a single tonie.
+type ScaffoldResult struct {
+	HouseholdID string
+	TonieID     string
+	Name        string
+	Folder      string
+	Created     bool // false if a matching folder already existed and was left untouched
+}
+
+// Scaffold creates the "<root>/<household_id>/<name> [id=<tonie_id>]" folder
+// structure for every household and Kreativ-Tonie on the account. Existing
+// tonie folders (matched by id, regardless of the current display name) are
+// left untouched so local audio files are never affected.
+func Scaffold(client ScaffoldClient, root string) ([]ScaffoldResult, error) {
+	households, err := client.GetHouseholds()
+	if err != nil {
+		return nil, err
+	}
+
+	var results []ScaffoldResult
+	for _, h := range households {
+		householdPath := filepath.Join(root, h.ID)
+		if err := os.MkdirAll(householdPath, 0o755); err != nil {
+			return nil, fmt.Errorf("creating household folder %s: %w", householdPath, err)
+		}
+
+		existingByID, err := existingTonieFoldersByID(householdPath)
+		if err != nil {
+			return nil, err
+		}
+
+		tonies, err := client.GetCreativeTonies(h.ID)
+		if err != nil {
+			return nil, err
+		}
+
+		for _, t := range tonies {
+			if existingDir, ok := existingByID[t.ID]; ok {
+				results = append(results, ScaffoldResult{
+					HouseholdID: h.ID,
+					TonieID:     t.ID,
+					Name:        t.Name,
+					Folder:      filepath.Join(householdPath, existingDir),
+					Created:     false,
+				})
+				continue
+			}
+
+			dirName := DirName(t.Name, t.ID)
+			tonieFolder := filepath.Join(householdPath, dirName)
+			if err := os.MkdirAll(tonieFolder, 0o755); err != nil {
+				return nil, fmt.Errorf("creating tonie folder %s: %w", tonieFolder, err)
+			}
+			results = append(results, ScaffoldResult{
+				HouseholdID: h.ID,
+				TonieID:     t.ID,
+				Name:        t.Name,
+				Folder:      tonieFolder,
+				Created:     true,
+			})
+		}
+	}
+	return results, nil
+}
+
+// existingTonieFoldersByID maps tonie id -> existing folder name within a household folder.
+func existingTonieFoldersByID(householdPath string) (map[string]string, error) {
+	entries, err := os.ReadDir(householdPath)
+	if err != nil {
+		return nil, fmt.Errorf("reading household folder %s: %w", householdPath, err)
+	}
+	byID := make(map[string]string)
+	for _, e := range entries {
+		if !e.IsDir() {
+			continue
+		}
+		if _, id, ok := ParseTonieDir(e.Name()); ok {
+			byID[id] = e.Name()
+		}
+	}
+	return byID, nil
+}