Email: the Inkwell logo in the header

The header showed only a yellow dot before the site name. It now shows the
app icon, embedded in the message as an inline part (cid:inkwell-logo, the
HTML part becoming multipart/related) rather than linked: most clients block
remote images, the app fetches nothing (rule 164), and Gmail drops SVG and
data: images. The PNG is a copy of frontend/public/icon-192.png, shipped as
package data; a test keeps the two identical.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_mail_templates.py

@@ -1,5 +1,7 @@
+import re
 from email import message_from_bytes
 from email.policy import default
+from pathlib import Path
 
 from inkwell import mail_templates
 from inkwell.mailer import MailSettings, build_message
@@ -27,11 +29,19 @@ def test_text_from_settings_and_people_is_escaped_in_the_html():
 
 def test_the_html_fetches_nothing():
     """No remote images or fonts: the app sends with no outbound network, and a remote
-    image in an email is a tracking pixel."""
-    html = mail_templates.password_reset("Inkwell", "a@example.test", LINK).html
-    assert "<img" not in html
-    assert "src=" not in html
-    assert "@import" not in html
+    image in an email is a tracking pixel. The one image is the logo, carried inside
+    the message and named by cid."""
+    email = mail_templates.password_reset("Inkwell", "a@example.test", LINK)
+    assert re.findall(r'src="([^"]*)"', email.html) == [f"cid:{mail_templates.LOGO_CID}"]
+    assert "@import" not in email.html and "<link" not in email.html
+    assert email.images == {mail_templates.LOGO_CID: mail_templates.logo_png()}
+
+
+def test_the_email_logo_is_the_app_icon():
+    """A copy, so the package carries it; this keeps the copy from drifting."""
+    icon = Path(__file__).resolve().parents[1] / "frontend" / "public" / "icon-192.png"
+    assert mail_templates.logo_png() == icon.read_bytes()
+    assert mail_templates.logo_png().startswith(b"\x89PNG")
 
 
 def test_the_message_is_text_first_then_html():
@@ -39,5 +49,14 @@ def test_the_message_is_text_first_then_html():
     msg = build_message(cfg, "a@example.test", mail_templates.test_message("Inkwell", "https://x"))
     parsed = message_from_bytes(msg.as_bytes(), policy=default)
     assert parsed.get_content_type() == "multipart/alternative"
-    assert [p.get_content_type() for p in parsed.iter_parts()] == ["text/plain", "text/html"]
+    text, related = parsed.iter_parts()
+    assert text.get_content_type() == "text/plain"
+    # The HTML travels with the logo it shows: multipart/related, html first.
+    assert related.get_content_type() == "multipart/related"
+    html, logo = related.iter_parts()
+    assert html.get_content_type() == "text/html"
+    assert f"cid:{mail_templates.LOGO_CID}" in html.get_content()
+    assert logo.get_content_type() == "image/png"
+    assert logo["Content-ID"] == f"<{mail_templates.LOGO_CID}>"
+    assert logo.get_content() == mail_templates.logo_png()
     assert parsed["Subject"] == "Inkwell test email"