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>

src/util/i18n.cpp

@@ -226,6 +226,9 @@ void I18n::loadBuiltinEnglish()
         strings_["chat_new_message"] = "Message";
         strings_["chat_new_send"] = "Send request";
         strings_["chat_cancel"] = "Cancel";
+        strings_["chat_add_contact"] = "Add contact";
+        strings_["chat_contact_added"] = "Contact added — rename it in Contacts";
+        strings_["chat_new_message_toast"] = "New encrypted chat message";
         strings_["chat_toast_not_connected"] = "Not connected — chat message not sent.";
         strings_["chat_toast_no_zaddr"] = "No z-address available to send chat from.";
         strings_["chat_toast_lite_busy"] = "A send is already in progress, or no wallet is open.";