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/library.go

@@ -0,0 +1,109 @@
+// Package library discovers Kreativ-Tonie mappings from a folder structure,
+// instead of a separate config file:
+//
+//	<root>/<household_id>/<freier_name> [id=<tonie_id>]/*.mp3
+//
+// The household id is the top-level folder name, and each tonie folder
+// name encodes both a free-form display name and the tonie id in brackets.
+// This keeps the "configuration" fully visible and editable as plain
+// folders on disk.
+package library
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+)
+
+// tonieDirPattern matches folder names like "Peppa Wutz [id=ef01-5678]".
+var tonieDirPattern = regexp.MustCompile(`^(?P<name>.+) \[id=(?P<id>[^\]]+)\]$`)
+
+// Tonie is a single discovered mapping between a local folder and a Kreativ-Tonie.
+type Tonie struct {
+	HouseholdID string // taken from the top-level folder name
+	TonieID     string // parsed from the "[id=...]" suffix of the folder name
+	Name        string // free-form part of the folder name (before " [id=...]")
+	Folder      string // absolute path to the folder containing the audio files
+}
+
+// ParseTonieDir parses a single tonie folder name into name and id.
+// Returns ok=false if the name doesn't match the expected "<name> [id=<id>]" pattern.
+func ParseTonieDir(dirName string) (name, id string, ok bool) {
+	m := tonieDirPattern.FindStringSubmatch(dirName)
+	if m == nil {
+		return "", "", false
+	}
+	return m[1], m[2], true
+}
+
+// DirName builds the folder name for a tonie: "<name> [id=<id>]".
+func DirName(name, id string) string {
+	return fmt.Sprintf("%s [id=%s]", sanitizeName(name), id)
+}
+
+// sanitizeName strips path separators and other characters that are unsafe in folder names.
+func sanitizeName(name string) string {
+	replacer := strings.NewReplacer("/", "-", "\\", "-", ":", "-")
+	return strings.TrimSpace(replacer.Replace(name))
+}
+
+// Discover walks root and returns all tonie mappings found in it.
+// Non-matching entries (files, folders without the "[id=...]" suffix) are skipped.
+func Discover(root string) ([]Tonie, error) {
+	householdEntries, err := os.ReadDir(root)
+	if err != nil {
+		return nil, fmt.Errorf("reading library root %s: %w", root, err)
+	}
+
+	var tonies []Tonie
+	for _, hEntry := range householdEntries {
+		if !hEntry.IsDir() {
+			continue
+		}
+		householdID := hEntry.Name()
+		householdPath := filepath.Join(root, householdID)
+
+		tonieEntries, err := os.ReadDir(householdPath)
+		if err != nil {
+			return nil, fmt.Errorf("reading household folder %s: %w", householdPath, err)
+		}
+
+		for _, tEntry := range tonieEntries {
+			if !tEntry.IsDir() {
+				continue
+			}
+			name, tonieID, ok := ParseTonieDir(tEntry.Name())
+			if !ok {
+				continue
+			}
+			tonies = append(tonies, Tonie{
+				HouseholdID: householdID,
+				TonieID:     tonieID,
+				Name:        name,
+				Folder:      filepath.Join(householdPath, tEntry.Name()),
+			})
+		}
+	}
+	return tonies, nil
+}
+
+// Filter returns only the tonies matching the given filter string, matched
+// case-insensitively as a substring against household id, tonie id, and name.
+// An empty filter returns all tonies.
+func Filter(tonies []Tonie, filter string) []Tonie {
+	if filter == "" {
+		return tonies
+	}
+	filter = strings.ToLower(filter)
+	var matched []Tonie
+	for _, t := range tonies {
+		if strings.Contains(strings.ToLower(t.HouseholdID), filter) ||
+			strings.Contains(strings.ToLower(t.TonieID), filter) ||
+			strings.Contains(strings.ToLower(t.Name), filter) {
+			matched = append(matched, t)
+		}
+	}
+	return matched
+}