v0.12.22: Billing suite — invoices, contact persons, and tabbed customer view

Invoices in Draft or Sent status can now be emailed directly from WarmDesk. Click Send by Email on any invoice row to open a compose dialog. The To field and greeting are pre-filled from the customer’s primary contact (see below). The message body supports Markdown — bold, italic, bullet lists, and links — with a live Preview tab so you can see the rendered email before sending. WarmDesk sends a multipart email with a plain-text fallback and the invoice PDF attached.

The pencil icon on every invoice opens an Edit Lines dialog where you can change descriptions, quantities, and unit prices for any existing line. You can also add new manual lines (quantity × unit price) or comment lines — full-width italic rows used for notes such as "On 2026-06-09 only charged 2 hours due to client being unprepared". The invoice subtotal, VAT, and total recalculate automatically on save.

Instead of a one-click status change, Mark Paid now opens a Record Payment dialog where you enter the payment date, amount received, and an optional reference number (bank transaction ID, cheque number, and so on). The date and reference are shown on the invoice row so you always have a quick audit trail.

For invoices that have already been sent or paid, an Issue Credit Note button creates a matching negating invoice — all line amounts are flipped — with status Credit Note. The original invoice gains an amber badge showing the credit note number. Credit notes that were issued in error can be voided (deleted) with Void Credit Note. Credit note PDFs show Credit Note as the document heading rather than Invoice.

A per-customer VAT-exempt flag suppresses the VAT row in PDFs entirely and inserts the locale-appropriate statutory note ("BTW verlegd", "Reverse charge", and so on) for all 12 supported languages.