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_unbez_akonto.sql

@@ -0,0 +1,20 @@
+-- =============================================
+-- Description:	Dashboard widget 'wdg_unbez_akonto' (Unbezahlte Akontos älter als 5 Arbeitstage (ohne MFR !)).
+--              Called from [dbo].[fis_widgets].[sql] as: SELECT * FROM [dbo].[fis_wdg_unbez_akonto]()
+-- =============================================
+CREATE FUNCTION [dbo].[fis_wdg_unbez_akonto]()
+RETURNS TABLE
+AS
+RETURN
+(
+    SELECT [Rechnung] = i.[invoiceId]
+         , [Name] = i.[InvoiceTitle] + CHAR(10) + '  (' + (SELECT TOP (1) c.[name] FROM [dbo].[mfr__companies] AS c WHERE i.[customerid] = c.[id]) + ')'
+         , [Betrag (net)] = FORMAT(i.[invoicebalance], N'#0.00€', 'de')
+         , [Datum] = FORMAT(i.[DateFinalized], 'dd.MM.yy', 'de')
+         , [Zahlungen] = FORMAT(ISNULL(pay.[Amount], 0.0), N'#0.00€', 'de')
+         , [order] = ROW_NUMBER() OVER (ORDER BY i.[DateFinalized], i.[DateSent])
+      FROM [dbo].[fds__invoices] AS i
+           LEFT JOIN [dbo].[fds__fn_InvoicePaymentAmounts]() AS pay ON pay.[InvId] = i.[id]
+     WHERE i.[InvoiceType] = 'i' AND i.[IsPayed] = 0 AND i.[iscanceled] = 0
+       AND i.[DateFinalized] < [dbo].[date_add](GETDATE(), '-5wd', 0)
+);