Remove tel: links when converting signature to plain version

[ceskyDJ]

Describe the bug

When I let the editor convert my signature (I think similar situation is for the whole message) from HTML to plain version, it doesn't remove tel: links, so it produces something like: "Phone: +420 777 123 456 tel:+420 777123456", which is ugly and I need to manually remove the tel:+420 777123456 part. It works well for e-mail (e.g. mailto:foo@example.com) links, so I hope it won't be such a problem to fix it for phone links, too.

To Reproduce (simpler way to describe point-by-point but doesn't work in New email modal, too)
Steps to reproduce the behavior:

1. Login to some account (in public interface, not admin one)
2. Go to Settings --> Edit identity --> Signature
3. Set HTML mode
4. Fill in Phone: <a href="tel:+420777123456">+420 777 123 456</a>
5. Set plain mode
6. See "Phone: +420 777 123 456 tel:+420 777123456" instead of just "Phone: +420 777 123 456"

Expected behavior
Remove tel: links before converting to plain text version, so "tel:+XXXXXXXXXXXX" won't be added.

Please complete the following information:

• Browser: Version 1.68.134 Chromium: 127.0.6533.88 (Official Build) (64-bit)
• IMAP daemon: dovecot (Mailcow)
• PHP version: 8.2.22
• SnappyMail Version: 2.37.2 (waiting for Watchtower to update it for testing purposes, shouldn't be fixed in the current release)
• Mode: Docker Compose

[the-djmaze]

It could be because of spaces or other tel: vs text markup.

For example: tel:+31612345678 an text +31 (0)6 1234 5678

Also some applications convert numbers to phonenumbers automatic (like whatsapp).

So what would be the best approach to convert these types of links?
As they are way more complex then email addresses.

[ceskyDJ]

Minimal change should consists in ignoring spaces when comparing href attribute and <a> content, as spaces aren't allowed in tel: links (see https://datatracker.ietf.org/doc/html/rfc3966#section-3).

The best results would be done by determining when there is a phone number in <a>'s content (e.g. "+420 777 123 456" vs "Call me"). But it's hard to check, so maybe the "minimal change" would be good enough.

[ceskyDJ]

Thanks!