Unify and improve "copy to clipboard" interactions

Is your feature request related to a problem? Please describe.

In many place a user can copy something to a clipboard.

Currently they have the issues of:

• Being implemented separately (logic, error handling, checking for support)
  • e.g., https://github.com/nextcloud/calendar/pull/8617/changes#r3620025970
• UI/UX inconsistencies
  • in some places we show success notifications
    • e.g., https://github.com/nextcloud/calendar/pull/8617/changes#r3620018238
  • in some places we use button changes
    • e.g., usages of https://nextcloud-vue-components.netlify.app/#/Components/NcFormBoxCopyButton

Describe the solution you'd like

1. Decide on consistent UI/UX patterns
2. Use consistent UI/UX patterns
3. Encapsulate UI/UX patterns in reusable components
4. Share copy-handling code between components

Describe alternatives you've considered

No response

Additional context

Having shared logic, we could write more elaborate copy-handling code supporting more browsers.

Simple module exposing an async copy function that uses the Async Clipboard API (navigator.clipboard) in secure contexts (HTTPS / localhost), with automatic fallback to document.execCommand('copy') for non-secure contexts or older browsers.

https://www.npmjs.com/package/copy-to-clipboard

Might use contribute back to https://github.com/nextcloud-libraries/nextcloud-vue/blob/f460a5c48898a9cebc656923b9c0ac4ffb969851/src/composables/useCopy.ts

Changing the button icon in place for a few seconds in case of success seems to be a very common pattern.
If no icon is used button text can change from "Copy text" to "Copied!" for a second or so.

Start by auditing the existing clipboard usages in the calendar app and compare them with nextcloud-vue's src/composables/useCopy.ts and NcFormBoxCopyButton. The issue does not name calendar files or tests, so first identify the affected components and agree on the UI pattern. Done means copy handling and feedback are shared and consistent across those interactions.