Rewrite toni-sync in Go

- Replace the Python implementation with a Go module compiling to a
  single static binary (no Python/pip runtime dependency)
- internal/tonieapi: minimal, dependency-light HTTP client for the
  TonieCloud REST API (login, households, creative tonies, file
  upload via presigned S3 request, chapter add/sort/clear)
- internal/config: YAML-based mapping of local playlist folders to
  Kreativ-Tonies, credentials never stored
- internal/syncer: diff/apply logic (upload new tracks, prune removed
  chapters, reorder to match local file order), built against a
  TonieClient interface for testability
- cmd/toni-sync: Cobra CLI with `tonies list`, `config add/list/remove`,
  `sync [NAME|--all] [--dry-run]`
- Unit tests for config persistence and syncer plan/apply logic

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

internal/config/config.go

@@ -0,0 +1,102 @@
+// Package config manages the local mapping between playlist folders and
+// Kreativ-Tonies. Credentials are never stored here.
+package config
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"gopkg.in/yaml.v3"
+)
+
+// Mapping maps a local folder (e.g. an exported Deezer playlist) to a Kreativ-Tonie.
+type Mapping struct {
+	Name        string `yaml:"name"`
+	HouseholdID string `yaml:"household_id"`
+	TonieID     string `yaml:"tonie_id"`
+	TonieName   string `yaml:"tonie_name,omitempty"`
+	Folder      string `yaml:"folder"`
+	PlaylistRef string `yaml:"playlist_ref,omitempty"`
+	Prune       bool   `yaml:"prune"`
+}
+
+// Config is the full toni-sync configuration.
+type Config struct {
+	Mappings []Mapping `yaml:"mappings"`
+}
+
+// Get returns the mapping with the given name, if present.
+func (c *Config) Get(name string) *Mapping {
+	for i := range c.Mappings {
+		if c.Mappings[i].Name == name {
+			return &c.Mappings[i]
+		}
+	}
+	return nil
+}
+
+// Upsert adds a new mapping or replaces an existing one with the same name.
+func (c *Config) Upsert(m Mapping) {
+	for i := range c.Mappings {
+		if c.Mappings[i].Name == m.Name {
+			c.Mappings[i] = m
+			return
+		}
+	}
+	c.Mappings = append(c.Mappings, m)
+}
+
+// Remove deletes the mapping with the given name. Returns false if it was not found.
+func (c *Config) Remove(name string) bool {
+	for i := range c.Mappings {
+		if c.Mappings[i].Name == name {
+			c.Mappings = append(c.Mappings[:i], c.Mappings[i+1:]...)
+			return true
+		}
+	}
+	return false
+}
+
+// DefaultPath returns the default config file location, honoring TONI_SYNC_CONFIG.
+func DefaultPath() string {
+	if p := os.Getenv("TONI_SYNC_CONFIG"); p != "" {
+		return p
+	}
+	home, err := os.UserHomeDir()
+	if err != nil {
+		home = "."
+	}
+	return filepath.Join(home, ".config", "toni-sync", "config.yaml")
+}
+
+// Load reads the config from path, or returns an empty Config if it doesn't exist yet.
+func Load(path string) (*Config, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return &Config{}, nil
+		}
+		return nil, fmt.Errorf("reading config %s: %w", path, err)
+	}
+	var cfg Config
+	if err := yaml.Unmarshal(data, &cfg); err != nil {
+		return nil, fmt.Errorf("parsing config %s: %w", path, err)
+	}
+	return &cfg, nil
+}
+
+// Save writes the config to path, creating parent directories as needed.
+func Save(cfg *Config, path string) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return fmt.Errorf("creating config dir: %w", err)
+	}
+	data, err := yaml.Marshal(cfg)
+	if err != nil {
+		return fmt.Errorf("encoding config: %w", err)
+	}
+	if err := os.WriteFile(path, data, 0o644); err != nil {
+		return fmt.Errorf("writing config %s: %w", path, err)
+	}
+	return nil
+}