Emit invoices as validated ZUGFeRD (DATEV) and XRechnung (B2G)

Completes the eRechnung output path: finalized invoices are emitted as a
ZUGFeRD/Factur-X EN 16931 hybrid (DATEV) or, when a Leitweg-ID is present, as
XRechnung 3.0 for B2G. Both are externally validated as ACCEPTED (0 errors) and
PDF/A-3B COMPLIANT against the ProcessWeb eInvoice service.

- ERechnungMapper: FdsInvoiceData -> EN 16931 model. Seller master data now from
  Fuchs:ERechnung:Seller config (VAT id BT-31, Steuernummer BT-32, optional
  Handelsregister BT-30, IBAN/BIC, contact). Adds payment terms BT-20/BT-9
  (BR-CO-25), buyer VAT id, §13b reverse charge, and the service date/period
  (BT-72 / BG-14) parsed from ProvisionPeriod (BR-DE-TMP-32). B2G -> XRechnung
  profile with the required electronic addresses/contact.
- ERechnungValidator: client for POST /validatepdf (EN 16931 XML + PDF/A-3 in
  one call). A pure "scenario not matched" with zero errors is not treated as a
  hard failure; real errors optionally withhold the hybrid (FailOnError).
- ERechnungSettings: seller + validation config; wired in Program.cs; ServiceUrl
  in appsettings.
- Online editor: structured German-only dialogs for the recipient address
  (incl. Leitweg-ID) and the service date/period (single date or range).
- Bumps the eRechnungLib submodule to the CII rsm-namespace / PDF-A subtype fix.

Fuchs.Tests 514/514, eRechnungLib 97/97.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Fuchs/appsettings.json

@@ -73,9 +73,23 @@
     "ERechnung": {
       "Enabled": false,
       "Profile": "EN16931",
+      "Seller": {
+        "Name": "Sebastian Fuchs GmbH & Co. KG",
+        "Street": "Germaniastraße 15",
+        "PostalCode": "40223",
+        "City": "Düsseldorf",
+        "CountryCode": "DE",
+        "TaxNumber": "106/5849/2962",
+        "VatId": "DE286366012",
+        "LegalRegistrationId": "",
+        "Iban": "DE76300501100045014800",
+        "Bic": "DUSSDEDDXXX",
+        "Email": "info@sanitaerfuchs.de",
+        "Phone": "0211 - 31 07 222"
+      },
       "Validation": {
-        "Enabled": false,
-        "ServiceUrl": "",
+        "Enabled": true,
+        "ServiceUrl": "https://api.processweb.de/api/eInvoice",
         "FailOnError": false
       }
     }