Implement PlayerStore wrapping signal<PlayerState> for player account data.
Adds applyLogin(raw) to fill state from login response A group, with:
- Mandatory fields get default 0/'' if missing (never undefined)
- Optional fields (openid/unionid/province/city) only set if present
- Immutable updates via spread operator
- Notifies subscribers on state change
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>