a later link drops a pending pubky sign-in request #1545

Observed

Bitkit holds only one deferred scan. Two cases drop a Pubky sign-in link:

1. A sign-in link opened on a cold start waits behind the PIN screen. A bitcoin: URI that arrives before unlock replaces it, and the sign-in request is lost:

   17:00:05.131 DEBUG [AppViewModel.kt] Replacing deferred scan from 'deeplink'
   

2. A payment URI that arrives while the Pubky Authorize sheet is open replaces that sheet with Send.

Seen while gating #1514 at d790f34. That PR does not change this behaviour.

Expected

A sign-in request already on screen, or waiting for unlock, is not discarded by a later link. The later link runs after the sign-in sheet closes, as it already does during the restore wait (Queuing 'deeplink' scan for deferred handling).

Steps to reproduce

1. Turn PIN on. Force-stop Bitkit and open a fresh pubkyauth://signin_grant link (adb shell am start -a android.intent.action.VIEW -d '<url>' to.bitkit.dev).
2. While the PIN screen is up, open bitcoin:<address> the same way.
3. Unlock. Send opens, and the sign-in request never appears.
4. Separately, with the Authorize sheet open, open bitcoin:<address>. The sheet is replaced by Send.

Root cause

AppViewModel.deferLockedScan / deferredScan hold a single slot that the next deferred scan overwrites. A non-mustComplete scan replaces the current sheet.

Suggested fix

Queue deferred scans in order, or keep a pending sign-in until it is handled. Do not replace an open Sheet.PubkyAuth with a payment scan; queue the scan until the sheet closes.