feat: Add reminder functionality to ContractSigner model

- Added `last_reminder_mail_sent_at` field to track reminder
  timestamps.
- Implemented `can_send_reminder` to check if a reminder can be
  sent based on a 30-minute cooldown.
- Added `reminder_cooldown_remaining` to get remaining cooldown
  time.
- Added `mark_reminder_sent` to update reminder timestamp.
- Added example demonstrating reminder functionality in
  `legal_contract_example.rs`.
- Added tests for reminder functionality in
  `test_reminder_functionality.rs`.
- Updated Rhai scripts to include reminder-related functions and
  tests.
- Improved formatting and clarity in example code.

specs/models/legal/contract.v

@@ -56,6 +56,7 @@ pub mut:
 	status     SignerStatus
 	signed_at  ourtime.OurTime // Optional in Rust, OurTime can be zero
 	comments   string        // Optional in Rust, string can be empty
+	last_reminder_mail_sent_at ourtime.OurTime // Unix timestamp of last reminder sent
 }
 
 // SignerStatus defines the status of a contract signer