Move fis widgets SQL into scalar functions

Add fis_wdg_* table functions for dashboard widgets and wire them into
FuchsDatabase.sqlproj; update the invoice list procs accordingly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Fuchs_Database/dbo/Functions/fis_wdg_invoices_overdue.sql

@@ -0,0 +1,14 @@
+-- =============================================
+-- Description:	Dashboard widget 'wdg_invoices_overdue' (Unbezahlte Rechungen Überfällig).
+--              Called from [dbo].[fis_widgets].[sql] as: SELECT * FROM [dbo].[fis_wdg_invoices_overdue]()
+-- =============================================
+CREATE FUNCTION [dbo].[fis_wdg_invoices_overdue]()
+RETURNS TABLE
+AS
+RETURN
+(
+    SELECT [label] = N'überfällige Rechnungen (10wd)', [value] = COUNT(*)
+      FROM [dbo].[fds__fn_unpaidInvoices]()
+     WHERE [dbo].[fds__fn_invoiceDate]([DateFinalized], NULL) >= DATEFROMPARTS(2026, 1, 1)
+       AND [dbo].[date_addcustom]([dbo].[fds__fn_invoiceDate]([DateFinalized], NULL), '10wd') < CAST(GETDATE() as date)
+);