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/ui/windows/chat_tab.h

@@ -4,6 +4,8 @@
 
 #pragma once
 
+#include <string>
+
 namespace dragonx {
 
 class App;
@@ -44,5 +46,11 @@ void RenderChatSettingsControls(App* app, float contentWidth = 0.0f, bool drawCa
  */
 void ResetChatTab();
 
+/**
+ * @brief Ask the Chat tab to open its New-conversation dialog prefilled with this z-address (consumed
+ *        on the next RenderChatTab frame). Used by the Contacts tab's "Message" action.
+ */
+void OpenChatNewConversation(const std::string& zaddr);
+
 } // namespace ui
 } // namespace dragonx