Fix the project's only compiler warning (unused `cardID` binding)

[eliseorobles]

Context

A full rebuild of every source and test file emits exactly one warning across the whole project:

Sources/ScanDeckApp/ScanDeckPresentationModel.swift:2775:19: warning: value 'cardID' was defined but never used; consider replacing with boolean test [#no-usage]

One permanent warning trains everyone to ignore the whole channel — which is exactly the channel a new contributor needs to trust. A clean build is the cheapest possible signal that a new warning means something.

What to change

Sources/ScanDeckApp/ScanDeckPresentationModel.swift:2774-2779:

var otherBlockingPairMismatchCount: Int {
    guard let cardID = cardPendingSizeAcceptanceConfirmation else {
        return 0
    }
    ...
}

Replace the binding with a nil test:

guard cardPendingSizeAcceptanceConfirmation != nil else {
    return 0
}

Behaviour is unchanged — the binding is never read.

Leave line 2782 alone. confirmSizeAcceptance() has a visually identical guard let cardID = cardPendingSizeAcceptanceConfirmation and does use it — it passes cardID to the reducer action. Changing that one breaks the operator's per-card size-mismatch override, which invariant #2 depends on.

How to verify

touch Sources/ScanDeckApp/ScanDeckPresentationModel.swift
swift build 2>&1 | grep warning     # should print nothing
swift test -c release               # 251 tests in 29 suites

See CONTRIBUTING.md — branch, run the suite, open a PR against main. No scanner needed.