feat(contacts,chat): a "Message" action linking Contacts to Chat

- Contacts: a "Message" context-menu item for shielded (z) contacts hands the
  address to the Chat tab (new OpenChatNewConversation entry point) and switches
  there with the New-conversation dialog prefilled; disabled with a reason for
  transparent-only contacts. (Kept to the safe context-menu + picker path, not
  the hand-drawn 4-slot row action cluster, to avoid mis-wiring the destructive
  Delete slot.)
- Chat: the New-conversation picker no longer silently hides transparent
  contacts — they're shown disabled with a "needs a shielded address to chat"
  tooltip instead of vanishing.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/i18n.cpp

@@ -300,6 +300,9 @@ void I18n::loadBuiltinEnglish()
         strings_["chat_unblocked_toast"] = "Conversation unblocked";
         strings_["chat_pick_contact"] = "Choose from contacts\xE2\x80\xA6";
         strings_["chat_no_z_contacts"] = "No shielded-address contacts yet";
+        strings_["chat_contact_needs_z"] = "Needs a shielded (z) address to chat";
+        strings_["contacts_message"] = "Message";
+        strings_["contacts_message_needs_z"] = "Chat needs a shielded (z) address — this contact only has a transparent one";
         strings_["chat_copy_address_tip"] = "Click to copy address";
         strings_["chat_verify_key"] = "Identity key \xE2\x80\x94 compare to verify";
         strings_["chat_awaiting_key"] = "Waiting for reply";