Merge branch 'signatures'

Signatures (item 152): one markdown file per signature under
~/.config/qtmaildir/signatures/, spliced into the composer buffer and
chosen from a switch on the editor bar. [compose] signature seeds a new
message, [account.<key>] signature overrides per account, and
[compose] signature_position picks end or above_quote.

Also carries three fixes found by hand-testing it: a saved draft is
indexed so it appears without a sync (item 158), the Drafts filter lists
messages rather than threads so a draft reply can be opened (item 159),
and a resumed draft no longer re-seeds its signature on a From: change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UUQS6n3cmsFrsjCNmwNtf8

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