fix: return typed cooldown errors and extend remittance timeout

[juliet-unoyiza]

Issues

Closes #1817
Closes #1816

Summary

• Replaces the lending-pool withdrawal cooldown panic with the typed PoolError::WithdrawalCooldownActive result and adds regression coverage.
• Preserves the existing ZeroShares error code for compatibility; the new error uses the next available code.
• Extends remittance transaction validity from 30 seconds to 300 seconds so users have enough time to review and sign in Freighter.

Validation

• Focused lending-pool regression test passed: test_immediate_withdraw_returns_error_when_cooldown_active.
• git diff --check passed.

[juliet-unoyiza]

@blurbeast Implemented both requested fixes in commit f85be79: #1817 now returns a typed withdrawal-cooldown error, and #1816 uses a 300-second transaction timeout. The PR body contains separate closing keywords for both issues.

[juliet-unoyiza]

@blurbeast I resolved the merge conflict with the latest main, preserving the typed cooldown error and 300-second signing timeout, and pushed the update. Please review and merge once the required checks are green.