fix(composer): keep HTML signature styling in the editor and on send

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>

components/email/quoted-html.ts

@@ -4,6 +4,8 @@ import { Node as TiptapNode, mergeAttributes } from "@tiptap/core";
 import { DOMSerializer } from "@tiptap/pm/model";
 import type { Editor } from "@tiptap/react";
 
+import { buildSignatureBlock } from "@/components/email/signature-block";
+
 // Marker attribute that identifies the quoted-original wrapper in serialized
 // HTML, so parseHTML can recognise it on the way back in.
 export const QUOTED_HTML_MARKER = "data-quoted-html";
@@ -166,6 +168,13 @@ export function serializeEditorContent(editor: Editor): string {
       parts.push(buildQuotedHtmlBlock((node.attrs.html as string) || ""));
       return;
     }
+    if (node.type.name === "signatureBlock") {
+      // Same rationale as quotedHtml: inline the verbatim signature HTML so the
+      // styled signature reaches the recipient (and a saved draft round-trips)
+      // instead of the schema-flattened version.
+      parts.push(buildSignatureBlock((node.attrs.html as string) || ""));
+      return;
+    }
     const fragment = serializer.serializeNode(node);
     const tmp = document.createElement("div");
     tmp.appendChild(fragment);