Add export button to sieve filter editor for providers without direct server access

Is your feature request related to a problem? Please describe.

Many email providers (including mine) do not offer Sieve management through the standard port 4190, but instead only provide a web interface for filter configuration. This makes Nextcloud Mail's automatic Sieve server connection unusable for us.

While I can create and manage filters perfectly using Nextcloud Mail's excellent GUI, I cannot deploy these rules to my actual email server because there's no way to access the generated Sieve code. The "Sieve script editor" is unavailable when no Sieve server is configured.

The specific problem: I can create rules visually in Nextcloud Mail, but I cannot transfer them to my email provider's web interface because there's no way to export or view the generated Sieve source code.

Describe the solution you'd like

I would like a way to access the generated Sieve code from the "Create a new filter" dialog. Either of these two approaches would solve the problem perfectly:

• Export button: Add an "Export" or "Save as" button next to the "Save" button that downloads the current rule as a .sieve text file
• Copy-paste display: Show the generated Sieve code in a read-only text field within the dialog for direct copying

Both options would be even better. 😉

Describe alternatives you've considered

Please refer to the solution description above, where I've already outlined two practical alternatives (export button or copy-paste display) that would solve this problem effectively.

All other alternatives I've considered, such as manual Sieve coding, switching providers, or using external tools, are significantly less practical for everyday use.

Additional context

I've already discussed this use case in issue #10655 where the maintainers suggested opening a separate feature request.

Many users face this limitation since numerous email providers only offer web-based Sieve management for security or simplicity reasons, while blocking direct port 4190 access.

This feature would make Nextcloud Mail's excellent filter editor accessible to a wider audience who currently cannot benefit from it due to provider restrictions.

Start at the “Create a new filter” dialog and trace how its visual rules are converted into Sieve source; compare this with the existing Sieve script editor path. Done means users without a configured Sieve server can view or export the generated rules from the dialog, with the chosen output behavior confirmed by tests.