Enhancement: Improve error message for unreachable Lightning Address

[jan3-agent]

Description

When sending to an invalid or unreachable Lightning Address, the agent returns a technical error message: LNURL-pay endpoint unreachable. This should be simplified to a user-friendly message.

Acceptance criteria

• Error message is simplified from technical jargon to plain language
• User guidance is provided (e.g., "check the address and try again")
• Message is consistent across all Lightning Address error paths

References

Related: #41 (QA test expectations)