feat: split envelopes by time of arrival

[GretaD]

fixes #11057

to do

• do not show an empty section

[ChristophWurst]

For reactivity: would it make sense to maintain the brackets in Pinia? Brackets relative to current time are probably not reactive. But if we commit the timestamp to Pinia, e.g. at every synchronization, and re-evaluate, it should be fine.

[st3iny]

There is a problem with an empty view initially. When the app loads there are no envelopes at first. Fetching initial envelopes is handled inside Mailbox.vue which is not rendered as there are no envelopes initially (and no groups as a consequence). As a result, no envelopes will be fetched and rendered until the first background sync hits.

My suggestions:

1. Move the sections/groups from MailboxThread.vue into Mailbox.vue.
2. Move some of the envelope fetching logic from Mailbox.vue to MailboxThread.vue (have a look at mounted(), loadEnvelopes() and prefetchOtherMailboxes() in Mailbox.vue).

In any case, you should rebase against latest origin/main soon as I implemented a lot of changes to the sync logic in Mailbox.vue.