feat(compose): render markdown bodies with cmark-gfm, item 123

The composer's body is markdown and the text/html part is generated from it.
cmark-gfm rather than plain cmark for autolink: under CommonMark a bare URL
in a mail body is not a link, and in mail it is expected to be clickable.

Three extensions are enabled and tables are deliberately not, since they
render badly across mail clients whoever generates them. Raw HTML in the
input is suppressed with CMARK_OPT_SAFE: the body is the user's own text,
but a body that can inject markup into its own generated HTML part is a
sharp edge with no upside.

The build needs TWO lookups. Only the core library ships a pkg-config file;
libcmark-gfm-extensions has none and is located with find_library, the way
notmuch already is. All three extensions live in that second library, so
finding only the first produces a build that compiles and silently renders
plain CommonMark.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015muoUo2GdxmBDSp5vjYcbE

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