Two invoice-PDF changes from #794.
1. VAT / free-text note (Benedikt's request, placement A). A new
`crm_invoices_vat_note_text` setting (Settings → CRM → Invoices) prints a
free-text line directly under the MwSt. row on every invoice. Data-driven:
the admin types the exact wording (Austrian Kleinunternehmer § 6 Abs. 1 Z 27
UStG, German § 19, reverse-charge, …) — no jurisdiction hardcoded. The
totals-block reserve grows by the measured note height so a long note can't
push the grand total into the footer. Read in invoice/render.js, threaded
through normaliseContext, drawn in drawTotals. Empty → row omitted; quotes
unaffected.
2. Multi-page footer overlap. On a full continuation page the line-item table
filled to the bottom margin, but the "Seite X von Y" stamp was drawn at
marginBottom-12 — INSIDE that fill zone — so items overlapped the page
number. Move the stamp into the bottom margin (below the content edge),
zeroing that page's bottom margin during the write so it can't trigger
PDFKit's auto-page-break. Verified: on a full page the lowest item text is
at pdfkitY ~790 while the page number sits at ~816 — ~26pt clearance.
Tests: render the note on a single page (byte-delta proves it renders) and
paginate a long invoice with the note (2–3 pages, no stray blank page).
Luca