TODO:
- for invoice importing using AI: all address data should be editable because import is not perfect
- for invoice importing using AI: file path should not be editable as it is imported
- create invoice PDF for NL https://goedestartbelastingdienst.nl/wiki/view/50bdccd8-f9a0-4297-b57f-3a6651cbe05c/factuureisen
- toggle on invoice form wether price is inclusive of tax.
- retrieve available balance from AI api & show in settings/services.
- let user choose the model to use in settings/services/ai
- real error logging for OpenAI and importing in general
- write tests for strops.hpp
- log_set_depth function so data can be grouped
- log elapsed time for ai requests
- refactor _add functions to use _import functions
- write tests that check error handling for corrupt files. (e.g. references to tax rates, project and cost center that failed to load)
- it is possible a referenced tax rate is loaded after an invoice is loaded. This means all invoices need to be recalculated after file load. (try to write a test for this).
- invoice sequential number should be modifyable & checked for uniqueness (for external invoices being imported)
- allow cost centers to be deleted that have 0 references
- Send invoice by email
- create invoice from image of receipt
- create invoice from UBL file
- read invoice from Holodeck instance
- send invoice via Holodeck instance
- View invoice history for contacts
- View invoice history for projects 
- validate data within administration on save to make sure it is valid for transmissions. (e.g. rules of https://docs.peppol.eu/poacc/billing/3.0/syntax/ubl-invoice/cac-AccountingSupplierParty/cac-Party/cbc-EndpointID/)
- View local business number / vat number naming on contact form. e.g. when country is austria: "Österreichische Umsatzsteuer-Identifikationsnummer"
- ICP reports