feat: implement Flexi-Save deposit/withdraw and unit tests

[Folex1275]

Description

This PR implements the core Flexi-Save functionality for the Nestera savings contract. This includes the logic for deposits and withdrawals, state synchronization with user records, and a comprehensive test suite to ensure financial accuracy and security.

Closes #7

🚀 Key Features Implemented

• Flexi-Save Module: Created the logic for users to deposit and withdraw funds with real-time balance updates.
• Dual-State Synchronization: Implemented a system to update both the specific FlexiBalance vault and the global User total balance record simultaneously.
• Security Checkpoints: Integrated require_auth to ensure only account owners can move their funds.
• Financial Validation: Added logic to prevent zero/negative deposits and block withdrawals that exceed the current balance.

🛠️ Technical Refactors (Fixes)

• Resolved naming collisions in lib.rs regarding the get_user function.
• Restored and standardized the User struct in storage_types.rs.
• Corrected module imports and crate scoping to support the new flexi and users modules.

🧪 Quality Assurance

• Total Tests: 36
• Result: All Passed ✅
• Focus: Specifically verified the Flexi-Save flow, ensuring that deposit_flexi and withdraw_flexi correctly modify the on-chain state and handle InsufficientBalance errors as expected.