feat(settings): add anniversary ICS subscription controls

[elig-45]

Summary

Add the frontend controls needed to create and copy a private anniversary calendar subscription URL.

This is the final frontend step for the anniversary ICS feature. Persistent token status and revocation remain centralized in the Access tokens manager from #1007.

What's included

• Add an Anniversary calendar (ICS) section under Settings > User with Gramps Web API 3.21 or newer.
• Generate or rotate the scoped anniversaries_ics token using:
  • POST /api/users/-/access-tokens/anniversaries_ics/
• Build the complete subscription URL:
  • /api/anniversaries.ics?token=...
• Allow the URL to be copied immediately after generation.
• Explain that the URL cannot be recovered later and must be regenerated if lost.
• Synchronize generation with the centralized token status from feat(settings): add persistent access token management #1007.
• Clear the displayed URL when the token is revoked centrally.
• Keep token revocation exclusively in the centralized Access tokens manager.

Security

• The raw token is used only to construct the subscription URL returned to the user.
• The component does not expose or store a separate raw token value.
• Existing token values are never retrieved.
• The generated URL is cleared after centralized revocation.
• No duplicate revocation logic or DELETE request is added here.

Testing

Added unit coverage for:

• API compatibility gating
• Token generation and rotation
• Complete and correctly encoded ICS URLs
• Same-origin and configured API hosts
• Missing-token and API failure handling
• Clipboard behavior
• Centralized revocation synchronization
• Absence of duplicate revocation behavior

Related

• Persistent token infrastructure: gramps-web-api#790
• Centralized persistent token management UI: #1007
• Anniversary ICS API endpoint: gramps-web-api#792
• Related issue: #683