Fix Inno Setup compile error and release script sed pattern

- Declare ErrorCode variable in InitializeSetup function
- Remove orphan var block at end of Code section
- Fix sed pattern in release.sh to only match version inside quotes
- Restore AboutForm.cs variable name damaged by previous sed

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

installer/setup.iss

@@ -72,6 +72,8 @@ begin
 end;
 
 function InitializeSetup(): Boolean;
+var
+  ErrorCode: Integer;
 begin
   Result := True;
 
@@ -81,11 +83,8 @@ begin
               'Soll die Download-Seite geoeffnet werden?',
               mbConfirmation, MB_YESNO) = IDYES then
     begin
-      ShellExec('open', 'https://dotnet.microsoft.com/download/dotnet/8.0/runtime', '', '', SW_SHOWNORMAL, ewNoWait, ResultCode);
+      ShellExec('open', 'https://dotnet.microsoft.com/download/dotnet/8.0/runtime', '', '', SW_SHOWNORMAL, ewNoWait, ErrorCode);
     end;
     Result := False;
   end;
 end;
-
-var
-  ResultCode: Integer;