Option to use Budget Flow as an expense-only tracker

Some of us only want to track what we spend, not what we earn. Right now that is not really possible: if you never enter income, the account balance drifts permanently negative, which pollutes the Overview, the net worth chart and the balance widgets. The current workaround is to enter income anyway with the Evaluated flag turned off, but that means maintaining data I never wanted to track in the first place.

I am not asking to restructure the app into separate income and expense sections (that has been requested and rejected before). A single setting per budget book would be enough, for example an “Expense-only mode”, which would hide the income transaction type in the entry form, hide balance-based widgets and the net worth chart from the Overview, and keep budgets and expense statistics working exactly as they do today.

No change to the data model would be needed - income would simply stop being surfaced. Budgets already work independently of account balances, so most of the app is unaffected.