Rich, table-based identity signatures lost all their inline CSS
(background/text colors, fonts, border-radius, bgcolor). The composer
embeds the signature into the TipTap editor, and parsing it into the
ProseMirror schema flattened it to a generic bordered table. That
normalized version was then shown while composing AND delivered to the
recipient, even though Identity settings stored and previewed it
correctly.
Hold the signature as a dedicated, non-editable atom node
(SignatureBlock) that keeps the verbatim HTML in an attribute and renders
it inside a Shadow Root, mirroring the existing QuotedHtml island. The
markup is never parsed into the schema, so the styling survives 1:1 both
in the in-editor preview and in the outgoing mail
(serializeEditorContent inlines the verbatim HTML, as it already does for
quoted originals). The signature stays a single unit: select it and
Backspace/Delete to remove it; identity switching still swaps it via the
existing data-signature-block markers.
Adds unit coverage (parse + serialize round-trip preserves inline styles).
Fixes#475
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
