feat(family_wallet): implement spending limit precision and rollover validations

[Hallab7]

Add enhanced spending limit system with precision controls and secure rollover
behavior to prevent over-withdrawal due to precision or period reset edge cases.

New Features

Enhanced Data Structures

• Add PrecisionSpendingLimit with min_precision, max_single_tx, and rollover controls
• Add SpendingPeriod for UTC-aligned daily period management
• Add SpendingTracker for cumulative spending across periods
• Extend FamilyMember with optional precision_limit field
• Add 5 new error types for precision validation failures

Core Security Enhancements

• Dust Attack Prevention: min_precision validation prevents micro-transactions
• Single Transaction Limits: max_single_tx prevents large withdrawals
• Overflow Protection: saturating_add() operations prevent arithmetic overflow
• Rollover Security: UTC boundary alignment prevents timezone manipulation
• Period Validation: rollover validation prevents time manipulation attacks

New API Functions

• set_precision_spending_limit() - Configure enhanced limits (Owner/Admin only)
• validate_precision_spending() - Comprehensive validation with rollover checks
• get_spending_tracker() - Read-only access to spending data
• Enhanced withdraw() function with precision validation integration

Security Assumptions

• Precision limits prevent dust attacks via configurable minimum amounts
• Period boundaries align to UTC midnight to prevent timezone exploitation
• Cumulative tracking resets only on legitimate period rollover
• Configuration validation ensures secure parameter relationships
• Owner/Admin roles bypass precision checks (unlimited spending)

Documentation

• Enhanced family-wallet-design.md with security assumptions
• Comprehensive API reference with NatSpec-style comments
• Migration guide and configuration examples
• Performance and storage efficiency considerations

Closes #293