feat(ux): copy-to-clipboard confirmations + scale wizard buttons for HiDPI

Feedback-convention + HiDPI cleanup (from the audit's living-hygiene threads):
- Copy actions that were silent now toast a confirmation: transaction
  address/txid (context menu, click-to-copy, detail buttons), peer and banned-
  peer addresses, the Settings data-dir copy, the chat message "Copy", and the
  key-export result copy.
- The first-run wizard's Skip/Back buttons used raw ImVec2(120/80,0) widths that
  rendered tiny at HiDPI; they now multiply by ui::Layout::dpiScale().

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

src/ui/windows/chat_tab.cpp

@@ -1738,7 +1738,10 @@ void RenderChatTab(App* app)
                         }
                     }
                     if (ImGui::BeginPopup("##msgmenu")) {
-                        if (ImGui::MenuItem(TR("copy"))) ImGui::SetClipboardText(m.body.c_str());
+                        if (ImGui::MenuItem(TR("copy"))) {
+                            ImGui::SetClipboardText(m.body.c_str());
+                            Notifications::instance().success(TR("copied"));
+                        }
                         ImGui::EndPopup();
                     } else if (s_msgmenu_index == static_cast<int>(mi)) {
                         s_msgmenu_index = -1;   // popup closed