Feature: read receipts (MDN, RFC 8098)

Bulwark had no read-receipt support (JMAP/Stalwart have no native MDN).
End-to-end, client-side, in three parts:

- Request (compose): a toolbar toggle (MailCheck, green when on) sets
  Disposition-Notification-To on the outgoing message via the JMAP
  "header:<name>:asText" create property. Threaded composer -> page ->
  email-store -> client.sendEmail. Default from requestReadReceiptDefault.

- Detect (viewer): reads Disposition-Notification-To case-insensitively from
  the parsed headers and shows a banner (green Send / red Ignore) in the
  unified notification bar. Hidden in Sent/Drafts/Trash/Junk and once handled.
  message/disposition-notification + message/delivery-status report parts are
  filtered out of the attachment list.

- Respond (MDN): lib/mdn.ts builds an RFC 8098 multipart/report (text/plain +
  message/disposition-notification, UTF-8/base64, localized subject + body).
  client.sendReadReceipt uploads the blob, imports it into Sent via
  Email/import, then submits with an explicit envelope. Both Send and Ignore
  set the $MDNSent keyword (RFC 3503) so no client re-prompts. Behaviour
  configurable: ask / always / never.

New: lib/mdn.ts, read-receipt-banner.tsx. Settings (requestReadReceiptDefault,
readReceiptResponse) + UI. All 17 locales.

locales/ja/common.json

@@ -260,6 +260,15 @@
     "reschedule_prompt": "新しい日時を入力してください。例: 2026-05-04T15:30"
   },
   "email_viewer": {
+    "read_receipt": {
+      "prompt": "送信者が開封確認を求めています：",
+      "send": "確認を送信",
+      "ignore": "無視",
+      "sent": "開封確認を送信しました。",
+      "send_failed": "開封確認を送信できませんでした",
+      "mdn_subject": "開封済み: {subject}",
+      "mdn_body": "これは、あなたが {recipient} に送信したメッセージの開封確認です。\n\n注意: この確認は、メッセージが受信者のコンピューターに表示されたことを示すだけです。受信者が内容を読んだ、または理解したことを保証するものではありません。"
+    },
     "no_email_selected": "メールが選択されていません",
     "no_email_description": "リストからメールを選択して表示してください",
     "no_conversation_selected": "会話が選択されていません",
@@ -545,6 +554,8 @@
     "undo_send": "送信を取り消す"
   },
   "email_composer": {
+    "read_receipt_on": "開封確認を要求中（クリックで無効化）",
+    "read_receipt_off": "開封確認を要求する",
     "new_message": "新規メッセージ",
     "reply": "返信",
     "reply_all": "全員に返信",
@@ -1006,6 +1017,17 @@
       }
     },
     "email_behavior": {
+      "request_read_receipt": {
+        "label": "既定で開封確認を要求する",
+        "description": "新しいメッセージの作成時に、開封確認の要求をあらかじめ有効にします。"
+      },
+      "read_receipt_response": {
+        "label": "開封確認の要求に応答する",
+        "description": "受信メッセージが開封確認を求めたときの動作。",
+        "ask": "毎回確認する",
+        "always": "常に送信",
+        "never": "送信しない"
+      },
       "title": "メール動作",
       "description": "メールの処理方法を設定",
       "mark_read": {