fix(notifications): skip logo attachment for conversational mails

The conversational mail template does not reference cid:logo.png, but
RenderMail still attached the embedded logo to every outgoing mail.
That left an orphan inline part that some clients render as a stray
attachment. Only embed logo.png when the formal template is in use.

pkg/notifications/mail_render.go

@@ -407,9 +407,12 @@ func RenderMail(m *Mail, lang string) (mailOpts *mail.Opts, err error) {
 		HTMLMessage: htmlContent.String(),
 		Boundary:    boundary,
 		ThreadID:    m.threadID,
-		EmbedFS: map[string]*embed.FS{
+	}
+
+	if !m.conversational {
+		mailOpts.EmbedFS = map[string]*embed.FS{
 			"logo.png": &logo,
-		},
+		}
 	}
 
 	return mailOpts, nil

pkg/notifications/mail_test.go

@@ -533,6 +533,7 @@ func TestConversationalMail(t *testing.T) {
 		// Should NOT have logo (completely removed)
 		assert.NotContains(t, mailopts.HTMLMessage, "logo.png")
 		assert.NotContains(t, mailopts.HTMLMessage, "Vikunja")
+		assert.NotContains(t, mailopts.EmbedFS, "logo.png")
 
 		// Should have inline action link with arrow
 		assert.Contains(t, mailopts.HTMLMessage, "View Task →")
@@ -571,6 +572,7 @@ func TestConversationalMail(t *testing.T) {
 		// Should HAVE logo in formal emails
 		assert.Contains(t, mailopts.HTMLMessage, "logo.png")
 		assert.Contains(t, mailopts.HTMLMessage, "Vikunja")
+		assert.Contains(t, mailopts.EmbedFS, "logo.png")
 
 		// Should have formal button styling
 		assert.Contains(t, mailopts.HTMLMessage, "background-color: #1973ff")