Add token approval / spending limits to wallet contract

[Miracle656]

Summary

Right now any signed assertion authorizes any operation. Add per-key spending limits or daily caps as an extra safety layer.

Files

• contracts/invisible_wallet/src/lib.rs
• contracts/invisible_wallet/src/storage.rs

Acceptance criteria

• New storage entry: (asset, daily_limit, used_today, last_reset)
• __check_auth() reads the limit and rejects if exceeded
• Daily reset logic
• Tests covering: under limit, at limit, over limit, day boundary