feat: update application name and user references to "Umschläge"

- Changed application name from "Minimal UI" to "SignFlow" in config-global.ts
- Updated navigation titles and paths from "User" to "Umschläge" in nav-config-dashboard.tsx
- Modified page title in user.tsx to reflect "Umschläge"
- Changed placeholder text in user-table-toolbar.tsx from "Search user..." to "Nach Umschlag suchen..."
- Updated headings and button labels in user-view.tsx to use "Umschläge" and "Neuer Umschlag"
- Commented out NavUpgrade component in nav.tsx
- Removed unused product and blog navigation items in nav-config-dashboard.tsx
- Added envelope-service.ts to handle envelope data with a basic structure
- Updated yarn.lock with dependency version changes

EnvelopeGenerator.GeneratorAPI/ClientApp/envelope-generator-react-ui/src/pages/user.tsx

@@ -7,7 +7,7 @@ import { UserView } from 'src/sections/user/view';
 export default function Page() {
   return (
     <>
-      <title>{`Users - ${CONFIG.appName}`}</title>
+      <title>{`Umschläge - ${CONFIG.appName}`}</title>
 
       <UserView />
     </>