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/contacts_tab.cpp

@@ -18,6 +18,7 @@
 #include "../../util/platform.h"
 #include "image_picker.h"
 #include "transactions_tab.h"
+#include "chat_tab.h"           // OpenChatNewConversation — Contacts "Message" action
 #include "../sidebar.h"
 #include "../layout.h"
 #include "imgui.h"
@@ -537,6 +538,15 @@ void RenderContactsTab(App* app)
         SetTransactionsAddressFilter(book.entries()[s_selected_index].address);
         app->setCurrentPage(ui::NavPage::History);
     };
+    // Message a contact in Chat — only z-contacts (chat needs a shielded address to carry encrypted
+    // memos). Mirrors doViewTransactions: hand the address to the Chat tab and switch there.
+    auto doMessage = [&]() {
+        if (!selValid()) return;
+        const auto& e = book.entries()[s_selected_index];
+        if (!isShieldedAddr(e.address)) return;
+        OpenChatNewConversation(e.address);
+        app->setCurrentPage(ui::NavPage::Chat);
+    };
 
     // Add/edit form — a modal popup layered over the tab, with a live contact preview and an avatar
     // picker (default Z/T badge, a Material icon, or a custom image via the in-app image picker).
@@ -1465,6 +1475,14 @@ void RenderContactsTab(App* app)
     if (ImGui::BeginPopup("##contactCtx")) {
         if (ImGui::MenuItem(TR("copy_address")))     doCopy();
         if (ImGui::MenuItem(TR("view_transactions"))) doViewTransactions();
+#if DRAGONX_ENABLE_CHAT
+        {
+            const bool canMsg = selValid() && isShieldedAddr(book.entries()[s_selected_index].address);
+            if (ImGui::MenuItem(TR("contacts_message"), nullptr, false, canMsg)) doMessage();
+            if (!canMsg && ImGui::IsItemHovered(ImGuiHoveredFlags_AllowWhenDisabled))
+                material::Tooltip("%s", TR("contacts_message_needs_z"));
+        }
+#endif
         if (ImGui::MenuItem(TR("edit")))             openEdit();
         ImGui::Separator();
         if (ImGui::MenuItem(TR("delete")))           doDelete();