feat: validate contract initialization state

[Kkkakania]

Fixes #34.

Summary

• add a verify_initialized helper that checks the required admin storage slot
• call initialization validation before each write operation mutates state
• add unit coverage for initialized and uninitialized contract validation

Notes

The issue mentions ContractError::NotInitialized, but this contract currently uses
panic strings for all error paths. This keeps the existing style and uses
not initialized for failed initialization validation.

Testing

• Baseline cargo test before changes -> 4 tests passed
• TDD red check first: cargo test failed because verify_initialized did not exist
• cargo fmt -- --check
• cargo test
• git diff --check