feat(contracts): add loktin-common crate for storage TTL and ID counters

[Peolite001]

Description

Closes #38

This PR addresses the issue of duplicated TTL constants and inconsistent ID counter behaviors across the contracts by introducing a new loktin-common crate.

Changes Made

• New Workspace Crate: Created crates/loktin-common.
• TTL Helpers: Added extend_ttl_instance and extend_ttl_persistent helpers using the existing ledger constants (DAY_IN_LEDGERS * 30 and DAY_IN_LEDGERS * 365).
• 1-Indexed ID Counter: Implemented a generic next_id helper that guarantees IDs start at 1 and monotonically increase. Added tests verifying this behavior.
• Deduplication: Removed localized TTL constants from the five contracts (spend_save, target_savings, plans, mock_pool, and locked_in), substituting them with the shared constants imported from the new crate.

Acceptance Criteria Met

• crates/loktin-common builds as a workspace member.
• Instance TTL extension helper added and callable.
• 1-indexed ID counter helper added.
• Duplicated TTL constants removed from the five contracts in favour of the crate.
• Tests added covering counter monotonicity and that the first ID issued is 1.

Notes

Adopting the newly added helpers inside each contract's entry points is kept out of scope for this PR, as outlined in the issue requirements.

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