Allow Züs-only allocations on Vult to be purchased with ZCN

Currently, users can only purchase Zus-only blobbers (through Vult/Blimp UI) with USD/Stripe. This hinders prevents privacy-conscious users from access a set of reliable blobbers. Using ZCN would bring back the token price and improve the tokenomics.

The primary goal would be to :
- BE: Automatically set the ZCN price for Züs-only allocation/blobbers, so it would have similar price as its USD amount. I.e if ZCN is at $0.0054, then the ZCN cost for a 50GB that costs $0.99/month would be around 1868ZCN. Potentially, we could give a slight discount when buying with ZCN instead of with USD. This can incentives people to use ZCN.
- FE : When purchasing or upgrading storage on the Vult Webapp, the "Pay with ZCN" should be available
This also ties into the mnemonic-only login option, but for now this would just be an option for those who have existing wallets

Note: Currently Zboxcli allows you to create allocations with zus-only blobbers with ZCN.

The issue involves backend pricing logic and frontend payment options in the Vult/Blimp UI. Start by examining the existing USD/Stripe payment flow for Züs-only allocations in the gosdk and the Vult webapp. Look for where allocation prices are set and where payment methods are selected. 'Done' means a user can select 'Pay with ZCN' in the UI, and the backend calculates the correct ZCN amount based on the current token price.

Scritto dal modello di indicizzazione a partire dal testo della issue.