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.