fix: give the Sent and Drafts views one row per message, in date order

Both views are lists of the user's own messages and are flat, but
walkThreads() emitted one ThreadSummary per THREAD and then picked a
single matched message to stand for it, breaking at the first one the
oldest-first walk reached. A conversation replied to twice therefore
produced one row: dated by the thread, opening the OLDER of the two
messages, with the newer one reachable nowhere in the view. Reported
against real mail, where a message sent at 12:42 was missing while the
row above it, dated 12:42, opened a message from three weeks earlier.

The same wrongly chosen message supplied firstMessagePath, so Delete or
Archive on such a row would have moved a file the user was not looking
at, silently, and mbsync would have carried it to the server. That half
was never visible.

The Sent branch now emits one summary per matched message, each carrying
its own id, tags, sender, path, date and subject. withRecipients still
selects the branch, so Sent and Drafts both get this and no second flag
can disagree with the flat-mode flag.

Ordering was a second defect under the same item, found by hand once the
rows appeared: notmuch_query_set_sort is a THREAD sort, so every row of a
thread inherits that thread's single position and an older reply drew
above a newer one. Sorting each thread's rows in place is not enough
either, since a message from another thread dated between them still
cannot land between them. Flat rows are collected and sorted as one list
before emitting.

ThreadListModel::rowKeyFor() is the second consequence and would have
broken quietly: two rows now share a threadId, and reconcile() keyed its
QHash on exactly that, so a sync would have dropped one of them by a
different route. It answers what makes a row unique, the message id in
flat mode and the thread id otherwise.

Tests cover both halves over new fixture threads F and G. The
cross-thread ordering assertion passed for the wrong reason at first,
because the existing fixture threads happen not to interleave; thread G
exists to break that and failed the moment it was added.
oldestFirstReversesTheOrder is corrected rather than satisfied:
OLDEST_FIRST orders threads by their oldest message while NEWEST_FIRST
orders by their newest, so the two lists mirror each other only while no
thread's date span contains another's, which this fixture is the first to
violate.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Jq9gXquUo9W4KXDagJXMmn

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