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>

go.mod

@@ -0,0 +1,15 @@
+module github.com/arnef/toni-sync
+
+go 1.26.4
+
+require (
+	github.com/spf13/cobra v1.10.2
+	golang.org/x/term v0.45.0
+	gopkg.in/yaml.v3 v3.0.1
+)
+
+require (
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/spf13/pflag v1.0.9 // indirect
+	golang.org/x/sys v0.47.0 // indirect
+)