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_today.sql

@@ -0,0 +1,12 @@
+-- =============================================
+-- Description:	Dashboard widget 'wdg_invoices_today' (Rechnungen Heute).
+--              Called from [dbo].[fis_widgets].[sql] as: SELECT * FROM [dbo].[fis_wdg_invoices_today]()
+-- =============================================
+CREATE FUNCTION [dbo].[fis_wdg_invoices_today]()
+RETURNS TABLE
+AS
+RETURN
+(
+    SELECT [label] = N'Rechnungen heute erstellt'
+         , [value] = FORMAT((SELECT COUNT(*) FROM [dbo].[fds__invoices] WHERE CAST([DateFinalized] as date) = CAST(GETDATE() as date)), '#0')
+);