feat(email): support multiple recipients for meeting summaries

[MasamiYui]

Summary

This PR enables users to configure multiple email recipients for automated meeting summaries. Recipients can now be specified as a comma-separated list in the application settings.

Changes

• Service : Updated EmailService.swift to parse and clean up comma-separated email addresses before sending.
• UI : Enhanced SettingsView.swift with updated labels, placeholders, and help text to guide users on multi-recipient configuration.
• Tests : Added comprehensive unit tests in EmailServiceTests.swift to verify recipient parsing logic (handling spaces, empty entries, etc.).
• Docs : Updated both English and Chinese documentation ( 10-email-notification.md , 10-email-notification.zh-CN.md ) and the feature specification.

Verification Results

• All unit tests passed ( swift test --filter EmailServiceTests ).
• Verified that the to field in the multipart request correctly formats multiple emails as a comma-separated string, trimmed string.