frontend: only pass account to Send component

[NicolaLS]

The Send component interacts with one account per transaction which is why it makes sense to refactor the component to only take one account instead of a list of all accounts and the code of the relevant account.

This has several benefits:

• reduce code size
• remove non-null assertions that should not be used (getAccount()!.coinCode for example)
• makes intention clear: Send can only take a valid account, if there is no account there is no Send.
• makes Send cleaner/easier to understand by removing prop, helper function, null/undefined checks.

Small PR working towards convert send.tsx to functional component. I will open a few small PR's and try to reduce the size of Send/make it easier to understand, and then eventually open a bigger PR to convert it to its own functional component.

But it will not be possible to do all refactoring before the big PR, for example a custom hook might be useful but can only be used when the component is functional itself.

[NicolaLS]

returning null if findAccount returns nothing feels lazy but is actually the graceful outcome of the Send component right now: https://github.com/BitBoxSwiss/bitbox-wallet-app/blob/87d6219ddb566a52aa410c54cd75d63669e5cec6/frontends/web/src/routes/account/send/send.tsx#L523-L526

But right now it could also crash because of the non-null assertion used.