[Enhancement] - Add option to email or SMS a receipt to the sender

[islandbitcoin]

After an invoice is paid, in addition to the print button, add a button to "Send Receipt".

This button will open a model that accepts email or phone number.

If an email is entered, the receipt will be sent to that email.

If a phone number is entered, the receipt will be sent to that phone number via SMS

If both are entered, the receipt will be sent to both email and phone number SMS.

We should use SendGrid as the email provider, and Twilio with templates as the SMS receipt provider.

[patoo0x]

📋 Definition of Done — 💰 50,000 sats

Expected Behavior After Fix:

• After invoice is paid, a "Send Receipt" button appears alongside the existing print button
• Tapping opens a modal accepting email, phone number, or both
• Email receipt sent via SendGrid with a clean, branded receipt format
• SMS receipt sent via Twilio with a concise receipt summary
• If both email and phone are entered, receipt is sent to both
• Input validation: valid email format, valid phone number format
• Success/error feedback shown to the merchant after sending

Deliverables:

• PR against main referencing this issue (closes #37)
• Unit tests for receipt generation, email send, SMS send, validation
• Screen recording showing: payment completes → tap Send Receipt → enter email → receipt arrives → enter phone → SMS arrives
• Lightning address provided in your first comment on this issue

Review Gate:

• All existing tests pass
• Nick signs off
• Dread final approval

Payment: 50,000 sats via Lightning on merge.
See CONTRIBUTING.md for full details.