The review of offline mode found ways to send a message twice, to lose
unsent mail without a word, and to send mail without its signing,
encryption or check plugin. This fixes them and the smaller findings:
- each outbox message is taken for sending in one database transaction
under a Web Lock per account; only a failure before the submission
request (SendNotStartedError from the client) puts it back, any other
one marks it "may have gone out"; read-backs after the submission no
longer fail a send; a send of a closed tab turns uncertain after a minute
- every sign-out the user didn't confirm names the deleted messages on
the sign-in page, or in a toast when another account stays signed in
- nothing is queued while an enabled plugin isn't running (after a start
without the network none can); the composer says why
- the outbox waits for a fresh policy and holds messages the rules have
since made stricter (a required label, a required send check)
- another tab switching offline mode off sticks: only switching it on
may create the database
- only attachments of kept messages are kept, filed with their message,
deleted with it and under the cap between syncs; file names and types
are sealed; folders that went away leave the scope, and a folder that
couldn't be asked keeps its mail without holding the others back
- a group scope that forbids offline mode waits until the account's
groups are known; one server down no longer holds another's mail
- an offline start only when the server is really unreachable, and the
real sign-in follows by itself once it answers
- the service worker files pages and chunks under the build that served
them, and the shell list of another build isn't kept under this one
- a raw send counts as the composer's only from the plugin running its
takeover, for that identity
- errors and refusals are in all languages; the banner tells "offline"
from "server can't be reached"; outbox buttons name their message;
changes and messages go in the order they were made