fix: show feedback when a scanned QR code contains unsupported data

[mvanhorn]

Contribution Summary

Linked Issue/Ticket: Refs #8520
RFC / Technical Design (if applicable): N/A

Description

The "Import via QR code" flow silently ignores QR codes whose payload fails validation. QrCodeScannerViewModel.handleQrCodeScanned records the payload hash in unsupportedPayloadHashes and returns with no state change, so the bottom bar keeps showing the generic help text and the user sees no reaction at all. That is the top-reported symptom in #8520: the camera clearly frames the code (it scans fine in other apps), but nothing happens because desktop exports with invalid payload data (blank authentication type, missing encryption settings) are decoded and rejected without feedback.

This adds a DisplayText.UnsupportedQrCode variant to QrCodeScannerContract, sets it in the unsupported branch of handleQrCodeScanned, and renders it in QrCodeScannerView.buildString with a new migration_qrcode_unsupported_code string. A subsequent successful scan restores the progress text through the existing handleSupportedPayload path. Only the base values/strings.xml is edited; translations flow through Weblate.

Scope note: this converts the silent failure into actionable feedback. It deliberately does not touch desktop-side QR generation (different repo) or device-specific camera lens selection, which the issue thread tracks separately, so the body references the issue with Refs rather than Closes.

Screen Shots

N/A: the change adds one line of text to the existing scanner bottom bar, shown only when an unsupported code is scanned.

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[github-actions]

✅ Validation Passed: All report and feature-flag labels are correctly set.

[dani-zilla]

Tested with a few QR codes, some working, others intentionally broken. Smoke test passed. However, it seems like 3 of the new tests are not:

> Task :feature:migration:qrcode:testDebugUnitTest FAILED

QrCodeScannerViewModelTest > user scans supported QR code after unsupported QR code FAILED
    kotlin.UninitializedPropertyAccessException at Log.kt:34
        Caused by: kotlin.UninitializedPropertyAccessException at QrCodeScannerViewModelTest.kt:164

QrCodeScannerViewModelTest > user scans unsupported QR code FAILED
    kotlin.UninitializedPropertyAccessException at Log.kt:34
        Caused by: kotlin.UninitializedPropertyAccessException at QrCodeScannerViewModelTest.kt:151

QrCodeScannerViewModelTest > user scans same unsupported QR code more than once FAILED
    kotlin.UninitializedPropertyAccessException at Log.kt:34
        Caused by: kotlin.UninitializedPropertyAccessException at QrCodeScannerViewModelTest.kt:180

[mvanhorn]

Good catch on the three — they were failing because Log.logger isn't initialized in the test, so the ViewModel's logging calls hit the uninitialized global and threw. Added Log.logger = TestLogger() in setUp(), matching the pattern the other feature tests use. I couldn't run the full Gradle suite locally, so worth a CI re-run to confirm all three go green.