feat(notifications): render bodies as Markdown

Balloon and drawer bodies switch from RichText to Qt's MarkdownText
(CommonMark plus GitHub extensions). Raw HTML still goes through Qt's
HTML importer, so body-markup senders keep working.

Markdown images fetch their destination the same way an <img> does,
which the existing sanitize filter would miss since it only inspects
tags. sanitize now escapes every "![" so the syntax renders as literal
text, local or remote; parsing CommonMark destinations and reference
definitions to allow local ones is not worth the grammar. Verified in
the shell log: before the filter reached the running shell, a test
body fetched https://example.org/a.png; after, a second one did not.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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