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>

go.sum

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