Remove is_chatmail and dependency on `XCHATMAIL` and `XDELTAPUSH` IMAP capabilities

[hpk42]

This issue servers as meta issue, and will/can be edited accordingly.

• base SMTP recipient chunking on relay IMAP maxsmtprecipients metadata, see use relay's maxsmtprecipients metadata for smtp chunking, to remove another is_chatmail use #8426

• Make a full scan through android, iOS and desktop code base for is_chatmail usage and determine replacement conditions

• Push notifications (XDELTAPUSH): Replace can_push() with can_metadata() check and try performing SETMETADATA /private/devicetoken.

  • Open question: What to do with Android's chatmail-conditional "do you want to enable permanent notifications" issue? (link2xt: as far as i understand from the comment below and looking at the code in remove DcAccounts.isAllChatmail() deltachat/deltachat-android#4549, Android simply assumes that if the user has FCM, then there is no need to ask for permanent notification even if some profiles are non-chatmail - that's fine, expert users who configure non-chatmail can also go to settings and enable permanent notification or don't really need instant notifications).

• Server-side deletion (XCHATMAIL): Collapse deletion logic in src/ephemeral.rs to depend on !bcc_self alone (deleting downloaded messages on single-device setups regardless of server type).

• Cleanup & Relay config: Remove is_chatmail from initial stats and info strings and whatever is left.

[r10s]

Make a full scan through android, iOS and desktop code base for is_chatmail usage and determine replacement conditions

we did that already in june at deltachat/deltachat-android#4480 , deltachat/deltachat-ios#3142 and deltachat/deltachat-desktop#6480

only remaining usage is whether to ask for "disable battery optimisations" or not (chatmail is considered to support PUSH, so we check if all-is-chatmail and if not, ask wrt battery optimisations). there is a proposal for removal that step at deltachat/deltachat-android#4549
EDIT: also the last occurrence is resolved