Pressing "Join with link" with the preview card already on screen previews the same ticket again #824

What is wrong

Pressing Join with link while the preview card for that same link is already on screen fetches the preview again.

• The button is plugins/social/client.js:1433: onClick: () => previewLink() — unconditional.
• previewLink (:1297-1326) always calls social/face-invite-preview, then replaces invite.preview with the fresh card.

So a second press costs another host round trip and resets the card's state (the countdown's clock offset, the status line), for a link the dialog is already showing. Reviewer2 flagged it on #35 as non-blocking.

Expected behaviour

When the card on screen is for the ticket currently in the field, pressing the button keeps that card and makes no preview call. A different ticket in the field previews once, and after Clear the next press previews once.

Acceptance

1. With a stubbed host counting social/face-invite-preview calls: two presses with the same ticket in the field = 1 call; the card stays up and its countdown is not reset.
2. Editing the field to a different ticket and pressing = a second call, once.
3. Clear, then press = one call.
4. The 404/timeout/unsupported paths keep today's behaviour (fall through to join, or stop with INVITE_UNSUPPORTED).
5. npm test in plugins/social green.

Kill conditions

• Comparing the raw URL string instead of the ticket: trailing whitespace or a different host for the same ticket would preview again.
• Disabling the button while a card is up: that removes the only way to re-check a link.
• Skipping the preview when the field's ticket changed (that is the opposite bug).
• Making the guard depend on busy only: the extra call happens when the dialog is idle.

Out of scope

• cleanRoomName's marker loop and Number(''); separate cleanup issues.
• The scan path: it calls previewLink(raw) with the scanned ticket, which is a fresh link by definition.