feat(config): read the three signature keys

[compose] signature and signature_position, and a per-account signature
that OVERRIDES the former. The account seeds the choice rather than owning
it: the composer's switch keeps every signature reachable whichever account
is selected, which is what keeps the note's "not tied to an account"
constraint intact.

The fallback is deliberately NOT resolved here. An account with no key of
its own carries an empty string, so the composer can tell "says nothing"
from "says none" and fall through itself.

signature_position follows quote_position's shape exactly, reporting a
present-but-malformed value rather than accepting it silently.

Part of item 152.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEcn3u19xPqv6ggD15PG4c

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