feat: pass app language to receipt PDF endpoint

[TaprootFreak]

Summary

• Forward the current UI language from SettingsBloc through DTOs, service, and cubits to the receipt PDF API endpoint
• The API uses this language override to generate receipts in the user's chosen app language instead of falling back to the user profile language
• Companion to DFXswiss/api#3865 which adds the backend support for the language parameter and the enhanced RealUnit receipt layout

Changes

• SingleReceiptDto / MultiReceiptDto: added optional Language? language field, serialized as uppercase code
• RealUnitPdfService: both getTransactionReceipt and getTransactionsReceipt accept and forward language
• TransactionHistoryReceiptCubit / TransactionHistoryMultiReceiptCubit: accept language in generateReceipt
• TransactionHistoryRow / TransactionHistoryDownloadButton: read language from SettingsBloc state and pass it to the cubit

Test plan

• All 47 existing transaction history tests pass
• Manual test: set app to German → download receipt → verify PDF is in German
• Manual test: set app to English → download receipt → verify PDF is in English
• Verify with API PR #3865 deployed