feat(invoices): offer proforma where the invoice is created (#2217) (#2254)

Proforma existed (document_type 'proforma', its own conversion path and
views) but the only way to pick it was the collapsed Förval panel inside
the editor. A user coming from Fortnox looked for it next to "Ny faktura",
did not find it, and concluded the feature was missing.

"Ny proformafaktura" is now an entry in the create split button on
/invoices, driven by ?proforma=1 exactly like ?quote=1 drives "Ny
offert": the URL opens NewInvoiceDialog with the proforma type
preselected, the dialog's accessible title says so, and closing the
dialog clears the param. Keys added to sv and en.

Closes #2217


Claude-Session: https://claude.ai/code/session_01VnConrmMCxJRQ5kfiPPWyy

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

messages/en.json

@@ -6465,6 +6465,8 @@
     "create_self_desc": "Register a self-billing invoice from your customer",
     "create_quote": "New quote",
     "create_quote_desc": "Create a quote that can become an invoice",
+    "create_proforma": "New proforma invoice",
+    "create_proforma_desc": "Advance invoice that is not booked and can become a real invoice",
     "bulkbar_selected": "{count, plural, one {invoice selected} other {invoices selected}}",
     "bulk_select_all": "Select all ({count})",
     "bulk_clear": "Clear",

messages/sv.json

@@ -6465,6 +6465,8 @@
     "create_self_desc": "Registrera en självfaktura från din kund",
     "create_quote": "Ny offert",
     "create_quote_desc": "Skapa en offert som kan bli en faktura",
+    "create_proforma": "Ny proformafaktura",
+    "create_proforma_desc": "Förhandsfaktura som inte bokförs och kan bli en riktig faktura",
     "bulkbar_selected": "{count, plural, one {faktura vald} other {fakturor valda}}",
     "bulk_select_all": "Markera alla ({count})",
     "bulk_clear": "Rensa",