Add Copy Meeting Link to event menu

[travisfortney]

What changed

• Rename the existing Copy event command to Copy Event Details.
• Add Copy Meeting Link when Itsycal detects a meeting URL.
• Keep the original shareable URL available for copying while preserving the
  existing app-specific Join behavior.
• Add localized strings for the updated commands.

Why

I use Itsycal every day and really appreciate the work that keeps it focused,
dependable, and native to macOS. Thank you to everyone who maintains it. This
change is intentionally small and follows the existing event and meeting-link
behavior.

The existing Copy command can already include a meeting URL when the URL is
stored in the event location. It also copies the event title and date and time,
however, so copying a link for use elsewhere requires removing that extra
content.

Copy Meeting Link puts only the detected meeting URL on the clipboard. For
services such as Zoom and Microsoft Teams, it copies the original shareable
HTTPS URL instead of an app-specific launch URL. The existing Join button
continues to open the installed meeting app when supported.

Screenshot

Testing

• Zoom URL in an event location
• Google Meet URL in an event location
• Microsoft Teams URL in an event location
• Meeting URL in the event URL field
• Meeting URL in event notes
• Ordinary event without a meeting URL
• Existing Copy Event Details output
• Existing Zoom Join button behavior
• Clipboard contains only the shareable meeting URL
• Clean local Debug build
• All existing localization files pass plutil -lint

The project has one application target and no automated test target, so this
small change was validated through the real event context-menu and clipboard
paths rather than adding a new test harness.