"Send failed: Invalid property or value." is Stalwart's description for
invalidProperties, and on its own it says nothing about what to fix. The
SetError also carries a `properties` array naming the offending fields, which
every call site was discarding.
setErrorMessage appends them, and the 35 places that surfaced a SetError -
send, save draft, mailboxes, calendars, contacts, sieve, files, signature
images, sharing - now go through it.
- Move the expand chevron into a gutter left of the folder icon so folders
with and without subfolders line up on their icon; labels share the column.
- Add "Mark all as read, incl. subfolders" to the folder context menu, with
the affected count and a per-folder fallback for servers without filter
operators.
- Re-measure the virtualised message list when row height changes.
- Mock: seed unread mail in a subfolder.