Fix: Auto-register wallet for existing users

[TaprootFreak]

Bug

When a user has multiple wallets or creates a new wallet, they may encounter the "Registrierung erforderlich" (Registration required) error on the Buy page, even though they already have completed registration with another wallet.

Root cause: Registration in the DFX system works on two levels:

• Account level: User data (name, address, email, etc.) - stored once per user
• Wallet level: Each wallet must be individually registered with DFX

When a user with existing account data tries to buy with a new/unregistered wallet, the API returns REGISTRATION_REQUIRED. Previously, the app would show an error message and require the user to manually go through the KYC/registration flow again - even though all their data is already in the system.

Solution

When the REGISTRATION_REQUIRED error is returned:

1. Check if user data already exists via getWalletStatus()
2. If userData exists → automatically call registerWallet() to register the current wallet
3. If successful → retry getting payment info (seamless UX)
4. If no user data exists or registration fails → fall back to showing the original error

This provides a seamless experience for existing users adding new wallets.

Changes

• BuyPaymentInfoCubit: Added _tryAutoRegisterWallet() method that attempts automatic wallet registration
• BuyPage: Added required service dependencies
• Tests: Updated mocks for new dependencies

Test plan

• Existing user with registered wallet: Buy flow works as before
• Existing user with new/unregistered wallet: Wallet is auto-registered, buy flow continues without interruption
• New user without account data: Shows "Registrierung erforderlich" as before

[xlamn]

Wrong approach by claude. Wait for #218 and then I do it myself.