panel: show the subject as text, not as its MIME encoding

A non-Latin subject arrives as RFC 2047 encoded-words, which the send-log
printed verbatim: unreadable, and one unbreakable run wide enough to push the
Status column out of its card. Decode at journal time (UTF-8/ASCII; exotic
charsets keep the raw header) and cap at 200 characters, then clip the column
to one line with the full text in the tooltip so no subject can widen the row.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/milter/milter_test.go

@@ -3,6 +3,7 @@ package milter
 import (
 	"errors"
 	"net"
+	"strings"
 	"testing"
 	"time"
 
@@ -113,6 +114,34 @@ func TestSessionRecordsRowPerRecipient(t *testing.T) {
 	}
 }
 
+// A subject in any non-ASCII alphabet reaches the milter as RFC 2047
+// encoded-words; the journal stores the text, not the encoding.
+func TestHeaderDecodesEncodedSubject(t *testing.T) {
+	long := strings.Repeat("я", subjectMaxRunes+10)
+
+	for _, tc := range []struct {
+		name, raw, want string
+	}{
+		{"plain", "Hello there", "Hello there"},
+		{"utf8 q", "=?utf-8?Q?=D0=9F=D1=80=D0=BE=D0=B2=D0=B5=D1=80=D0=BA=D0=B0?=", "Проверка"},
+		{"utf8 b, folded across two words", "=?utf-8?B?0J/RgNC40LLQtdGC?=\r\n =?utf-8?B?INC80LjRgA==?=", "Привет мир"},
+		// No decoder for the legacy single-byte charsets: keep the header as
+		// sent rather than losing the subject entirely.
+		{"unknown charset", "=?windows-1251?B?z/Do4uXy?=", "=?windows-1251?B?z/Do4uXy?="},
+		{"too long", long, strings.Repeat("я", subjectMaxRunes) + "…"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			s := &session{rec: &fakeRecorder{}}
+			if _, err := s.Header("Subject", tc.raw, mods(nil)); err != nil {
+				t.Fatalf("Header: %v", err)
+			}
+			if s.subject != tc.want {
+				t.Fatalf("subject = %q, want %q", s.subject, tc.want)
+			}
+		})
+	}
+}
+
 func TestBodyAcceptsEvenWhenRecorderFails(t *testing.T) {
 	rec := &fakeRecorder{fail: true}
 	s := &session{rec: rec}