feat(compose): save drafts atomically into a Maildir, item 123

DraftStore::write() renders a built message into <folder>/cur with the
given Maildir flags, writing through QSaveFile and unlinking the previous
revision only after the new file is in place.

Two orderings here are load-bearing and both are covered by a test that
was checked against the mutation that breaks it.

The unlink runs only after the write has succeeded, so a failed save
leaves the previous revision intact rather than losing both. Provoking
that failure needs care: the plan's version used an unwritable path where
mkpath() fails and the function returns before reaching either the write
or the unlink, so a mutation moving the unlink up survived it. The test
uses an existing but read-only cur/ instead, where the failure lands at
the write.

And the size comparison stays ahead of commit() in the condition, because
QSaveFile::commit() returns true after a short write and renames the
truncated bytes into place: measured, write 4096 of 65536 with commit
reporting true and the file left in the listing. What leaves the
directory empty is the short-circuit returning before commit() is
reached, after which ~QSaveFile() discards the uncommitted scratch file.
Reducing the condition to !file.commit() looks like a simplification and
writes a truncated draft into cur/, where notmuch would index it and
mbsync would upload it.

Resolving the mail root stays the caller's job, per item 124; this takes
an absolute folder path and composes nothing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QP2g3b3kuLx6AYFCNEz6UR

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