fix: preserve HTML signature when sending a quick reply

The quick-reply box built its body with appendPlainTextSignature, which runs
the identity's HTML signature through htmlToPlainText, and sent a text-only
message (htmlBody was undefined). A formatted signature (e.g. <strong>…) was
therefore flattened to plain text in the sent mail, even though it previewed
correctly in the identity editor. The full composer already builds an HTML
signature block; quick reply did not.

Add an appendHtmlSignature helper (mirrors the composer's send-time block) and,
when the sending identity has an HTML signature, send a matching HTML body from
handleQuickReply so the markup is preserved. Text-only identities keep the
plain-text-only behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/__tests__/signature-utils.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it } from 'vitest';
 
 import {
+  appendHtmlSignature,
   appendPlainTextSignature,
   getPlainTextSignature,
   hasMeaningfulHtmlBody,
@@ -27,6 +28,27 @@ describe('signature-utils', () => {
     });
   });
 
+  describe('appendHtmlSignature', () => {
+    it('appends a sanitized html signature, preserving formatting', () => {
+      expect(appendHtmlSignature('<div>Hello</div>', { htmlSignature: '<strong>Alice</strong>' }))
+        .toBe('<div>Hello</div><br><br>-- <br><strong>Alice</strong>');
+    });
+
+    it('escapes and appends a text signature when no html signature exists', () => {
+      expect(appendHtmlSignature('<div>Hello</div>', { textSignature: 'Alice\nEng' }))
+        .toBe('<div>Hello</div><br><br>-- <br>Alice<br>Eng');
+    });
+
+    it('omits the separator marker when disabled', () => {
+      expect(appendHtmlSignature('<div>Hi</div>', { htmlSignature: '<strong>A</strong>' }, { separator: false }))
+        .toBe('<div>Hi</div><br><br><strong>A</strong>');
+    });
+
+    it('leaves the body untouched when no signature exists', () => {
+      expect(appendHtmlSignature('<div>Hi</div>', {})).toBe('<div>Hi</div>');
+    });
+  });
+
   describe('hasMeaningfulHtmlBody', () => {
     it('prefers html bodies that preserve signature formatting', () => {
       expect(hasMeaningfulHtmlBody('<div>Hello</div><br><p>Alice</p>')).toBe(true);