feat(delete): move the message to the account's trash folder

Delete added the `deleted` tag and moved nothing, so deleted mail sat in
the inbox indefinitely with only a chip saying otherwise. It now moves the
file into the account's trash, records where it came from, and moves it
back on undo.

The origin is derived in the WORKER, not in the UI, because nowhere else
knows it. A Maildir filename does not record the folder a message came
from and notmuch cannot answer once the file has moved, so the moment the
old filename exists inside moveMessages() is the only place it can be
read. It travels back on a new messagesMovedFrom() signal, and the UI
turns it into a `deleted-from:<folder>` tag that Restore reads days later.

The account is resolved from the message's PATH rather than from its
account tag: that tag is optional config, so resolving through it would
silently make an account undeletable. That needed ThreadSummary to carry
the first message's path, since an unexpanded thread row is the ordinary
case and held no path at all. It is reported relative to the database
root, because the UI knows accounts only by their maildir, itself a
database-relative prefix.

accountForMessagePath() accepts both an absolute and a relative path, and
that is load-bearing rather than defensive: a thread row's path is
relative while a reply row's is absolute, since MimeParser has to open it.
Matching only one form left Delete on a reply resolving to no account and
moving nothing, which is the thread-row/reply-row asymmetry this file has
been bitten by before.

Tags are applied only once the worker CONFIRMS the move. Tagging first
would leave a message marked deleted in a folder it never left when a
rename fails, which is the half-done state this removes. A move made
during a sync is held in its own queue and flushed like a tag edit: the
existing queue carries tag changes only, so a move pushed through it would
apply `deleted` and never move the file.

An account with no trash configured reports through the status bar and
tags nothing, as a second line of defence behind the config-load warning.

Six existing tests used `delete` as a stand-in for a message-scoped tag
action on bare windows with no account; they move to `spam` and
`delete_thread`, which stayed tag-only, keeping the property each was
actually testing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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