feat(chat): copy, add-to-contacts, and new-message toast (Q2/Q3/Q4)

- Q3: copy the peer z-address from the thread header (SmallButton), and right-click
  any message to copy its body.
- Q2: an "Add contact" action in the header when the peer isn't already known —
  one click saves them to the address book (rename later in Contacts).
- Q4: capture ChatService::ingest's new-message count (previously discarded) and
  fire an in-app toast when new encrypted chat arrives while the user isn't on the
  Chat tab (main-thread MainCb sites only).

i18n (EN + 8 languages, additive; no new CJK glyphs).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

res/lang/ja.json

@@ -135,12 +135,15 @@
     "change_pass_title": "パスフレーズを変更",
     "characters": "文字",
     "chat": "チャット",
+    "chat_add_contact": "連絡先に追加",
     "chat_cancel": "キャンセル",
+    "chat_contact_added": "連絡先を追加しました — 連絡先で名前を変更できます",
     "chat_contact_request": "連絡リクエスト",
     "chat_empty_hint": "まだ会話はありません。受信したメッセージはここに表示されます。",
     "chat_locked_hint": "チャットを読み込むにはウォレットのロックを解除してください。",
     "chat_new_button": "新しい会話",
     "chat_new_message": "メッセージ",
+    "chat_new_message_toast": "新しい暗号化チャットメッセージ",
     "chat_new_send": "リクエストを送信",
     "chat_new_title": "新しい会話",
     "chat_new_zaddr": "宛先Zアドレス",