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_reminder_open10wd.sql

@@ -0,0 +1,18 @@
+-- =============================================
+-- Description:	Dashboard widget 'wdg_reminder_open10wd' (Offene Mahnungen, älter 10wd).
+--              Called from [dbo].[fis_widgets].[sql] as: SELECT * FROM [dbo].[fis_wdg_reminder_open10wd]()
+-- =============================================
+CREATE FUNCTION [dbo].[fis_wdg_reminder_open10wd]()
+RETURNS TABLE
+AS
+RETURN
+(
+    WITH rem AS (SELECT [invid], [rem#] = COUNT(*), [last] = MAX([DateFinalized]) FROM [dbo].[fds__reminder] AS r GROUP BY [invid])
+    SELECT [label] = N'ALTE offene Mahnungen (10wd, nur Intranet)', [value] = COUNT(*)
+      FROM [dbo].[fds__fn_unpaidInvoices]() AS i
+           LEFT JOIN rem ON i.[id] = rem.[InvId]
+     WHERE i.[fds] = 1
+       AND [dbo].[fds__fn_invoiceDate](i.[DateFinalized], NULL) >= DATEFROMPARTS(2026, 1, 1)
+       AND [dbo].[date_addcustom]([dbo].[fds__fn_invoiceDate](rem.[last], NULL), '10wd') < CAST(GETDATE() as date)
+       AND ISNULL(rem.[rem#], 0) > 0
+);