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_test.go

@@ -0,0 +1,74 @@
+package config
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+func TestUpsertAddsNewMapping(t *testing.T) {
+	cfg := &Config{}
+	m := Mapping{Name: "a", HouseholdID: "h1", TonieID: "t1", Folder: "/tmp/a"}
+	cfg.Upsert(m)
+	got := cfg.Get("a")
+	if got == nil || got.TonieID != "t1" {
+		t.Fatalf("expected mapping to be added, got %+v", got)
+	}
+}
+
+func TestUpsertReplacesExistingMapping(t *testing.T) {
+	cfg := &Config{}
+	cfg.Upsert(Mapping{Name: "a", HouseholdID: "h1", TonieID: "t1", Folder: "/tmp/a"})
+	cfg.Upsert(Mapping{Name: "a", HouseholdID: "h1", TonieID: "t2", Folder: "/tmp/a2"})
+
+	if len(cfg.Mappings) != 1 {
+		t.Fatalf("expected 1 mapping, got %d", len(cfg.Mappings))
+	}
+	if cfg.Get("a").TonieID != "t2" {
+		t.Fatalf("expected updated tonie id t2, got %s", cfg.Get("a").TonieID)
+	}
+}
+
+func TestRemoveReturnsFalseWhenMissing(t *testing.T) {
+	cfg := &Config{}
+	if cfg.Remove("nope") {
+		t.Fatal("expected Remove to return false for missing mapping")
+	}
+}
+
+func TestSaveAndLoadRoundtrip(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+
+	cfg := &Config{}
+	cfg.Upsert(Mapping{
+		Name:        "peppa",
+		HouseholdID: "h1",
+		TonieID:     "t1",
+		Folder:      "/tmp/peppa",
+		PlaylistRef: "https://example.com/playlist/1",
+		Prune:       true,
+	})
+
+	if err := Save(cfg, path); err != nil {
+		t.Fatalf("Save failed: %v", err)
+	}
+
+	loaded, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load failed: %v", err)
+	}
+	m := loaded.Get("peppa")
+	if m == nil || m.TonieID != "t1" || m.PlaylistRef != "https://example.com/playlist/1" {
+		t.Fatalf("unexpected loaded mapping: %+v", m)
+	}
+}
+
+func TestLoadMissingFileReturnsEmptyConfig(t *testing.T) {
+	cfg, err := Load(filepath.Join(t.TempDir(), "does-not-exist.yaml"))
+	if err != nil {
+		t.Fatalf("Load failed: %v", err)
+	}
+	if len(cfg.Mappings) != 0 {
+		t.Fatalf("expected empty mappings, got %+v", cfg.Mappings)
+	}
+}