fix: tighten typing on Card and Token dataclasses

[piitaya]

Summary

The Card, Chapter, Track, and Token dataclasses had broken annotations like field: str = None (assigning None to a non-Optional type) and dict[Track] = None (missing key type). mypy strict and pyright both flagged them, and downstream code (Home Assistant yoto integration) had to work around it with card.chapters or {} everywhere.

Changes

• Fix Optional annotations: str = None → str | None = None, same for int.
• Fix dict typing: dict[Track] → dict[str, Track], dict[Chapter] → dict[str, Chapter].
• Make identifier fields required (no default): Card.id, Chapter.key, Track.key. They're always set at construction by the parser, and a card/chapter/track without an identifier doesn't exist in the Yoto model.
• Default chapters and tracks to field(default_factory=dict) instead of None. A card always has chapters and a chapter always has tracks in the Yoto data model; the None was only an artifact of incremental parsing. Empty dict represents the "not yet loaded" state.
• Remove now-dead is None guards in client.py and or {} in the e2e test.

Impact

No runtime behavior change. Pure annotation tightening plus removal of dead guards.

Downstream consumers (HA core integration) can drop their card.chapters or {} defensive code and trust the type.