feat-658-google-calendar-reminders

[EcstaceeLOR]

Summary

Adds a complete calendar export flow for stream completion reminders.

Changes

• Adds a Google Calendar deep link with the stream completion timestamp and contract URL.
• Keeps iCalendar (.ics) export for Apple Calendar, Outlook, and other RFC 5545-compatible clients.
• Includes the stream contract URL in both calendar event descriptions.
• Adds unit coverage for Google Calendar URL construction and the component's two export options.

Verification

• Added focused unit tests for the calendar helpers and calendar button.
• Local test execution is pending because the environment cannot complete dependency installation without its certificate chain; this repository currently exposes no PR checks.

Closes #658

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