feat(invoices): configurable VAT/free-text note + fix multi-page page-number overlap (#794)

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).

frontend/src/i18n/locales/de.json

@@ -5233,6 +5233,11 @@
     "crm_invoices_late_fee_label": {
       "label": "Bezeichnung Mahngebühr"
     },
+    "crm_invoices_vat_note_text": {
+      "label": "MwSt.- / Freitext-Hinweis auf Rechnungen",
+      "placeholder": "z. B. Gemäß § 6 Abs. 1 Z 27 UStG 1994 wird keine Umsatzsteuer berechnet (Kleinunternehmer).",
+      "help": "Wird direkt unter der MwSt.-Zeile auf jeder Rechnungs-PDF gedruckt. Leer lassen zum Ausblenden. Bitte den genauen Wortlaut mit deinem Steuerberater abstimmen."
+    },
     "crm_invoices_skonto_percent_default": {
       "label": "Standard-Skonto %"
     },

frontend/src/i18n/locales/en.json

@@ -5231,6 +5231,11 @@
     "crm_invoices_late_fee_label": {
       "label": "Late fee label"
     },
+    "crm_invoices_vat_note_text": {
+      "label": "VAT / free-text note on invoices",
+      "placeholder": "e.g. Gemäß § 6 Abs. 1 Z 27 UStG 1994 wird keine Umsatzsteuer berechnet (Kleinunternehmer).",
+      "help": "Printed directly under the MwSt. line on every invoice PDF. Leave empty to hide. Please confirm the exact wording with your tax advisor."
+    },
     "crm_invoices_skonto_percent_default": {
       "label": "Skonto rate (default %)"
     },