Arithmetic expressions not allowed in balance assertions created with editor

[s-truax]

When using the "add transaction" function of the editor to add a balance assertion, arithmetic expressions are not considered valid input.

For example, the following balance assertion is well-formed and raises no errors with bean-check but can't be entered using the fava editor.

2024-10-01 balance Expenses:Fees (10 + 0.25) USD

See screenshot for another example:

This can probably be fixed by changing this line in fava/frontend/src/entry-forms /Balance.svelte

    <input
      type="tel"
      pattern="-?[0-9.,]*"
      placeholder={_("Number")}
      size={10}
      bind:value={entry.amount.number}
    />