TODO:

Fix:
- When reloading view, lock main thread as the freeing might cause a crash
- reload invoice when backing out of edit mode. Unsaved changed are not undone atm
- Handle missing file loading crash
- handle invalid file format crash. check if xml object is found before using it.
- invoice total is wrong when invoice is not using main currency (final is correct but currency tag is wrong)

Refactor:
- Refactor zip writing to be faster (Windows is slow)
- fix all warnings from Wall
- load ttfs from memory, embed ttfs in binary
- remove hardcoded buffer sizes from api provider implementations
- remove hardcoded buffer sizes from administration_writer.cpp

Testing:
- write tests for strops.hpp
- write tests that check error handling for corrupt files. (e.g. references to tax rates, project and cost center that failed to load)
- write tests for document saving

Features:
- Button to mark expense as paid
- Button to mark invoice as paid
- status window for ongoing requests, used can click on item when result is ready. Should be usefull for batch imports.
- implement gemini and perplexity api backends
- Refactor contact and project UI to be like invoice & expenses
- Handle invalid api key response from AI backends and display in settings UI
- error log for tax report to display invoices not yet supported for tax generation or invoices with invalid tax rates
- minimum invoice date for tax report generation, show warnings for unsupported invoices
- 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.
- 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