Add MSVC support and implement ECDSA key validation

[alienx5499]

Summary

This PR adds MSVC compiler support and implements basic ECDSA key validation functionality. The changes address existing TODO items and improve code quality.

Changes

MSVC Compiler Support

• File: CMakeLists.txt
• Change: Replaced TODO comment with MSVC compiler flags
• Details: Added comprehensive MSVC flags for warnings, C++ standards compliance, and code quality
• Impact: Enables Windows development and CI/CD support

ECDSA Key Validation

• File: src/crypto/key_validator/key_validator_impl.cpp
• Change: Implemented basic ECDSA key validation (replacing TODO comments)
• Details:
  • Private key validation: checks size (32-66 bytes) and prevents all-zero keys
  • Public key validation: checks size (33-133 bytes) and prevents all-zero keys
  • Returns appropriate error codes for invalid keys
• Impact: Addresses issue #103 and improves security

Code Quality

• File: src/storage/sqlite.cpp
• Change: Added const correctness to error handling methods
• Details: Made getErrorCode() and getErrorMessage() const methods

Testing

The ECDSA validation logic was tested with various key sizes and formats:

• Valid private key (32 bytes): PASS
• Invalid private key (16 bytes): PASS (correctly rejected)
• Zero private key (32 bytes): PASS (correctly rejected)
• Valid public key (33 bytes): PASS
• Invalid public key (16 bytes): PASS (correctly rejected)
• Valid public key (64 bytes): PASS

All tests passed successfully.

Technical Details

The MSVC flags include standard warning levels, C++ compliance settings, and code quality checks. The ECDSA validation follows conservative validation patterns - rejecting invalid keys rather than accepting them.

Files Changed

• CMakeLists.txt - Added MSVC compiler flags
• src/crypto/key_validator/key_validator_impl.cpp - Implemented ECDSA validation
• src/storage/sqlite.cpp - Added const correctness

Verification

• Code compiles without errors
• ECDSA validation logic verified with test cases
• No breaking changes to existing functionality
• Follows project coding standards

[alienx5499]

@turuslan Could you please review this PR when you get a chance?