feat(chat): add a "Copy my chat address" button

The conversation header only copies a PEER's address, so a user had no way to
hand out their own chat address for others to message them. Add a "Copy my chat
address" link in the left pane (public App::chatMyAddress() forwards the stable
seed-derived reply z-address; copied on click only, with an "address copied"
toast and a warning if none exists yet).

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

src/ui/windows/chat_tab.cpp

@@ -916,6 +916,24 @@ void RenderChatTab(App* app)
             if (ImGui::SmallButton(bl.c_str())) s_show_blocked = true;
             ImGui::PopStyleColor();
         }
+        // "Copy my chat address" — the address others use to reach this user. The conversation header
+        // only copies a PEER's address, so without this there's no way to hand out your own. Copies on
+        // click only (chatMyAddress may persist a freshly chosen address).
+        {
+            ImGui::PushStyleColor(ImGuiCol_Text, material::OnSurfaceMedium());
+            // Text-only: SmallButton uses the body font, which has no Material icon glyph (would show "?").
+            if (ImGui::SmallButton(TR("chat_copy_my_address"))) {
+                std::string myAddr = app->chatMyAddress();
+                if (myAddr.empty()) {
+                    Notifications::instance().warning(TR("chat_no_my_address"));
+                } else {
+                    ImGui::SetClipboardText(myAddr.c_str());
+                    Notifications::instance().info(TR("address_copied"));
+                }
+            }
+            ImGui::PopStyleColor();
+            if (ImGui::IsItemHovered()) material::Tooltip("%s", TR("chat_my_address_tip"));
+        }
         const std::string search = s_search;
         ImGui::Separator();
         if (convs.empty()) {