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.cpp

@@ -96,6 +96,8 @@ int composeInputCallback(ImGuiInputTextCallbackData* data) {
 bool s_show_new_convo = false;
 char s_new_zaddr[128] = "";
 char s_new_msg[256] = "";
+char s_pending_new_zaddr[128] = "";   // cross-tab request from Contacts' "Message" action
+bool s_pending_new_set = false;
 char s_search[80] = "";       // conversation-list filter (Q8)
 bool s_show_hidden = false;   // when on, the list also shows hidden conversations (with an Unhide action)
 bool s_show_delete_confirm = false;  // "Delete conversation?" confirm overlay (revive vs block)
@@ -766,6 +768,17 @@ void RenderChatTab(App* app)
         return;
     }
 
+    // Consume a cross-tab "Message this contact" request (from Contacts): open the New-conversation
+    // dialog prefilled with the z-address. Runs every frame the tab renders, so it fires right after the
+    // Contacts tab switches us here.
+    if (s_pending_new_set) {
+        s_pending_new_set = false;
+        s_show_new_convo = true;
+        std::memcpy(s_new_zaddr, s_pending_new_zaddr, sizeof(s_new_zaddr));
+        s_new_msg[0] = '\0';
+        sodium_memzero(s_pending_new_zaddr, sizeof(s_pending_new_zaddr));
+    }
+
     // Resolve the Chat-tab clock once per frame: the chat override wins, else the global format.
     if (auto* cst = app->settings()) {
         const int ctf = cst->getChatTimeFormat();   // 0=follow global, 1=24h, 2=12h
@@ -2020,17 +2033,27 @@ void RenderChatTab(App* app)
                 const std::string activeHash = app->activeWalletScopeId();  // per-wallet contact scope
                 int shown = 0;
                 for (const auto& e : book.entries()) {
-                    if (e.address.empty() || e.address[0] != 'z') continue;   // chat requires a z-address
+                    if (e.address.empty()) continue;
                     // Respect the same per-wallet scope the Contacts tab enforces — don't leak another
                     // wallet's scoped contact into this wallet's picker (global + legacy fail open).
                     const bool visible = e.isGlobal() ||
                         (e.scope.rfind("w:", 0) == 0 ? (!activeHash.empty() && e.scope == activeHash) : true);
                     if (!visible) continue;
-                    ++shown;
                     const std::string item = e.label + "   " + shorten(e.address, 16, 8);
-                    if (ImGui::Selectable(item.c_str())) {
-                        std::strncpy(s_new_zaddr, e.address.c_str(), sizeof(s_new_zaddr) - 1);
-                        s_new_zaddr[sizeof(s_new_zaddr) - 1] = '\0';
+                    // Show transparent contacts too, but DISABLED with a reason — chat needs a shielded
+                    // address, so silently hiding them left the user wondering where a contact went.
+                    if (dragonx::util::isShieldedAddress(e.address)) {
+                        ++shown;
+                        if (ImGui::Selectable(item.c_str())) {
+                            std::strncpy(s_new_zaddr, e.address.c_str(), sizeof(s_new_zaddr) - 1);
+                            s_new_zaddr[sizeof(s_new_zaddr) - 1] = '\0';
+                        }
+                    } else {
+                        ImGui::BeginDisabled();
+                        ImGui::Selectable(item.c_str(), false);
+                        ImGui::EndDisabled();
+                        if (ImGui::IsItemHovered(ImGuiHoveredFlags_AllowWhenDisabled))
+                            material::Tooltip("%s", TR("chat_contact_needs_z"));
                     }
                 }
                 if (shown == 0) {
@@ -2242,6 +2265,8 @@ void ResetChatTab()
     sodium_memzero(s_compose, sizeof(s_compose));
     sodium_memzero(s_new_zaddr, sizeof(s_new_zaddr));
     sodium_memzero(s_new_msg, sizeof(s_new_msg));
+    sodium_memzero(s_pending_new_zaddr, sizeof(s_pending_new_zaddr));
+    s_pending_new_set = false;
     sodium_memzero(s_rename_buf, sizeof(s_rename_buf));
     s_selected_cid.clear();
     s_scroll_to_cid.clear();
@@ -2268,6 +2293,13 @@ void ResetChatTab()
     s_msgmenu_index = -1;
 }
 
+void OpenChatNewConversation(const std::string& zaddr)
+{
+    std::strncpy(s_pending_new_zaddr, zaddr.c_str(), sizeof(s_pending_new_zaddr) - 1);
+    s_pending_new_zaddr[sizeof(s_pending_new_zaddr) - 1] = '\0';
+    s_pending_new_set = true;
+}
+
 void RenderChatSettingsControls(App* app, float contentWidth, bool drawCards)
 {
     auto* st = app ? app->settings() : nullptr;