feat: send invoices to recipients by email from Invora

[Navdeepannu]

Depends on: feat: add authentication and secure account-scoped invoice persistence

Goal

Add a Send invoice action that emails the invoice to the recipient from Invora and records the delivery attempt on the saved invoice.

User flow

1. A signed-in user opens a saved invoice.
2. The user selects Send invoice.
3. A dialog shows the recipient email, subject, optional message, and invoice attachment/link.
4. The user confirms the send.
5. Invora displays the result and records delivery metadata.

Scope

• Send button in the editor and saved-invoice detail flow.
• Recipient email prefilled from client information but editable before sending.
• Subject and optional message fields.
• Server-side PDF generation or a secure, time-limited invoice link.
• Server-only email provider integration.
• Reply-to set to the authenticated user's verified business email where supported.
• Delivery status, provider message ID, sent timestamp, and failure reason.
• Clear success, retry, and failure states.
• Rate limiting and duplicate-send protection.
• Email template that is readable on mobile and includes the invoice number, amount, due date, and sender identity.

Security and reliability requirements

• Keep email provider credentials server-only.
• Validate recipient addresses and invoice ownership on the server.
• Prevent arbitrary users from sending another user's invoice.
• Do not expose permanent public invoice URLs unless explicitly enabled.
• Use idempotency or an equivalent mechanism to reduce accidental duplicate sends.
• Configure and document sender-domain verification, SPF, DKIM, and DMARC requirements.
• Do not mark an invoice as sent until the provider accepts the request.