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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• This contribution is in Kotlin where possible
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• This PR has a descriptive title and body that accurately outlines all changes made, and contains a reference to any issues that it fixes (e.g. Closes #XXX or Fixes #XXX).

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