bug: a link opened during a pending pubky sign-in drops the sign-in #1542

Refs:

• bug: pubky sign-in link can still time out on cold start while paykit holds the shared-state lock #1541
• fix: keep pubky sign-in links working on a cold start #1514

What happened?

A link that arrives while a Pubky sign-in link is pending replaces the sign-in link. @jvsena42 saw both cases during the device test of #1514 (details):

• A bitcoin: URI opened while the PIN screen holds a deferred sign-in link replaces it (Replacing deferred scan from 'deeplink'), and the sign-in link is dropped.
• A URI opened while the Authorize sheet is open replaces the sheet with Send.

Expected behavior

A pending sign-in link is not dropped by a later link: the later link waits until the sign-in is approved or cancelled, as it already does while a sign-in link waits for the Pubky session.

Steps to Reproduce

1. Turn the PIN on, force-stop Bitkit and open a pubkyauth://signin_grant link.
2. While the PIN screen shows, open a bitcoin: URI.
3. Unlock: Send opens and the Authorize sheet never appears.
4. Open a sign-in link with Bitkit unlocked, then open a bitcoin: URI while the Authorize sheet is open: Send replaces the sheet.

Additional context

Both behaviours predate #1514; deferred scans keep a single slot.