The JMAP body-fetch path never requested or stored `bodyStructure`, so
`body.mimeTree` was always null for JMAP accounts — causing Show Mail
Structure to show nothing.
Fix: include `bodyStructure` in the JMAP `Email/get` request and convert
it to the same JSON format used by the IMAP path via the new
`_jmapBodyStructureToJson` helper. The parsed tree is persisted in the
DB and returned from `getEmailBody`, so the cached round-trip also works.
Tests added:
- Unit: JMAP getEmailBody populates mimeTree from bodyStructure and
survives the cache round-trip; null when bodyStructure is absent.
- Widget: Show Mail Structure dialog displays all MIME parts when
mimeTree is present; snackbar appears when mimeTree is null.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
