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/rich-text-editor.tsx

@@ -17,6 +17,7 @@ import { TableRow } from "@tiptap/extension-table-row";
 import { TableHeader } from "@tiptap/extension-table-header";
 import { TableCell } from "@tiptap/extension-table-cell";
 import { QuotedHtml, serializeEditorContent } from "@/components/email/quoted-html";
+import { SignatureBlock } from "@/components/email/signature-block";
 import { cn } from "@/lib/utils";
 import {
   Bold,
@@ -235,6 +236,10 @@ export function RichTextEditor({
       // Quoted/forwarded original email body - held verbatim as an atomic
       // node so layout-heavy HTML survives 1:1 (see quoted-html.ts).
       QuotedHtml,
+      // Identity signature - held verbatim as a non-editable atomic node so
+      // rich/branded signatures keep their inline styling in the editor and
+      // in the sent mail (see signature-block.ts).
+      SignatureBlock,
     ],
     content,
     editorProps: {