refactor: extract creator state storage keys into constants module

[MarcusDavidG]

Adds ADMIN_ADDRESS constant and creator()/key_balance() constructor functions to constants::storage. Updates all call sites in lib.rs and test.rs to use the shared constants instead of inline DataKey variants.

Closes #52

Summary

• Added ADMIN_ADDRESS constant to constants::storage
• Added creator() and key_balance() constructor functions to constants::storage for parameterised keys
• Updated all DataKey::Creator, DataKey::KeyBalance, and DataKey::AdminAddress call sites in lib.rs and test.rs to use the shared constants
• No behavior change introduced

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

Checklist

• Linked issue or backlog item
• Contract behavior and invariants are described clearly
• Docs updated if contract interfaces or workflows changed
• Scope stays limited to one contract concern

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