fix: enhance invoice download and electronic invoice processing

• Introduced a new JS script block in download.php which gets triggered when user session is not valid or on missing parameters invoice or type.
• Dialog.js has been modified to change how invoices are downloaded. Instead of creating an invisible iframe, now it leverages fetch API and download.js to get the invoice and proceed downloading.
• Corrected indentation of the text and button image in the dialog box of the invoice download.
• Invoice.php updated to include a new check for net value invoices. Adds articles to the ZugferdDocumentBuilder and calculates VAT based on the detected invoice type (net or gross). This ensures accurate electronic invoices.