fix: flag a saved draft seen so it is not tagged unread

DraftStore::write() was called with "D", and it uses the flag string
verbatim, so every draft this application wrote landed as :2,D. With
maildir.synchronize_flags on, notmuch tags any message lacking the S
flag `unread`, and a draft the user authored is seen by definition.

The symptom heals itself: the next sync of that folder round-trips the
file, adds S, and the tag goes away. Only the newest draft in a folder
that has not synced since shows it, which is why it read as
intermittent and why measuring an older draft finds nothing wrong.

TestComposeWindow::aSavedDraftIsFlaggedSeen() asserts both flags on the
written filename, verified failing first against "D".

TestMainWindow::anAutosaveWritesADraftAndClearsTheDirtyFlag() asserted
endsWith(":2,D"), pinning the whole flag set where its own comment said
the point was the draft flag "not left bare", so it failed against the
corrected behaviour. It checks for D within the flag set now.

Also reconciles the backlog with the user's notes: records the
forwarded-HTML defect as item 171, closes item 169 (shipped last
session, its row still read open and its section was still in the open
file), and records this fix as item 172.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AtUzfNjMD8fiYfamDd3ywW

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