refactor: use specific i18n error key for copy-invite-link failure in useMembersPanel

## Summary

In `src/platform/workspace/composables/useMembersPanel.ts`, the `handleCopyInviteLink` catch block uses the generic `t('g.error')` toast summary. This should be replaced with a dedicated, actionable i18n key so users understand what failed and what they can do next.

## Proposed Change

```diff
} catch {
toast.add({
severity: 'error',
- summary: t('g.error')
+ summary: t('workspacePanel.members.invites.copyLinkFailed'),
+ detail: t('workspacePanel.members.invites.copyLinkFailedAction')
})
}
```

- Add `copyLinkFailed` and `copyLinkFailedAction` keys to i18n locale files under `workspacePanel.members.invites`.
- Optionally log or report the caught error via the existing logger for observability.

## Context

Flagged during review of PR #11402 (test coverage PR) but deferred as the generic message is pre-existing and out of scope for that PR.

**Backlinks:**
- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11402
- Review comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11402#discussion_r3114698560

Requested by @christian-byrne

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11496-refactor-use-specific-i18n-error-key-for-copy-invite-link-failure-in-useMembersPanel-3496d73d36508170b16ae54a5ff48a45) by [Unito](https://www.unito.io)