Wallet: differentiate between transaction and message in KeystoneProvider.tsx

[Thoralf-M]

Description

The header

iota/apps/wallet/src/ui/app/components/keystone/KeystoneProvider.tsx

Line 134 in c45a160

<Header title="Confirm Transaction" titleCentered onClose={() => onCancel()} />

has Confirm transaction hardcoded, however this component is also used when signing a message (for example to login into a dApp) instead of a transaction. In this case it shouldn't mention transaction

Motivation

Make it clear that a message and not a transaction is signed for the case where it's a message.