feat(mail): sender display name (SMTP_FROM_NAME, default MindSift)

A bare SMTP_FROM address gets the display name in the From header; a name
already in SMTP_FROM wins, the envelope keeps the bare address. Also a
field in the admin mail settings.

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

internal/api/mail_test.go

@@ -270,3 +270,17 @@ func TestAuthLoginOnlyAndBothFailing(t *testing.T) {
 		t.Fatalf("expected both mechanisms and the server hint in the error, got %v", err)
 	}
 }
+
+func TestSenderAddressGetsDisplayName(t *testing.T) {
+	cases := []struct{ raw, name, envelope, header string }{
+		{"alerts@example.com", "MindSift", "alerts@example.com", `"MindSift" <alerts@example.com>`},
+		{`"Mein Dienst" <alerts@example.com>`, "MindSift", "alerts@example.com", `"Mein Dienst" <alerts@example.com>`},
+		{"alerts@example.com", "Büro Müller", "alerts@example.com", "=?utf-8?q?B=C3=BCro_M=C3=BCller?= <alerts@example.com>"},
+	}
+	for _, c := range cases {
+		env, hdr, err := senderAddress(c.raw, c.name)
+		if err != nil || env != c.envelope || hdr != c.header {
+			t.Fatalf("senderAddress(%q, %q) = %q, %q, %v", c.raw, c.name, env, hdr, err)
+		}
+	}
+}