fix: issue #256 - savings goals storage and target date validation

[menawar]

This PR resolves failing tests and implements strict target date validation for the savings_goals contract. close #256

Key Changes

• Storage Migration: Moved all core contract state (GOALS, NEXT_ID, OWNER_GOAL_IDS, etc.) from instance() to persistent() storage for better data reliability.
• Target Date Validation: Implemented strict checks in create_goal to ensure target dates are in the future and within a 10-year window.
• TTL Management: Added robust TTL management for all persistent keys, ensuring they are bumped appropriately during state changes.
• Initialization Correction: Fixed init() to start NEXT_ID at 0, ensuring consistent ID generation starting from 1.

Verification

• 77/77 unit tests in lib.rs passing.
• Stress tests for large amounts and high goal volume passing.
• TTL re-bump verification passing.
• Gas benchmarks within expected ranges.

[Baskarayelu]

@menawar Please resolve the conflicts