Support ECDSA auth scheme

### Feature description

Miden Wallet currently only supports the Falcon auth scheme. Now that the client also supports ECDSA, the wallet should expose that scheme as an option.

Most likely, the wallet should present an additional onboarding step for new account creation, to select the desired auth scheme. That auth scheme can remain for the lifetime of the account. Wallet backup will also need to record the auth scheme.

### Why is this feature needed?

_No response_

Look at the existing Falcon auth scheme implementation in the Miden Wallet codebase to understand the onboarding flow and account creation. Identify where the auth scheme selection is handled and where wallet backup records are updated. The work involves adding a new ECDSA option, modifying the UI for selection, and ensuring the scheme is persisted for the account's lifetime.

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