fix(chat): bound the selection copy range to the message body

The copy button of a text selection in a chat bubble copied
body.substr(start, len) with byte offsets kept from earlier frames.
Those offsets can outlive a thread change (an older message inserted
above shifts the message indices, so the selection can land on a
shorter body), and a start past the end made substr throw
std::out_of_range out of the chat render; the main loop catches it,
but the rest of that frame's UI is dropped.

The copy now runs only when the start is not past the end of the body.
substr already clamps the length, so the copied text is the same in
every case that did not throw before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/windows/chat_tab.cpp

@@ -1653,7 +1653,10 @@ void RenderChatTab(App* app)
                         if (cbhov) {
                             ImGui::SetMouseCursor(ImGuiMouseCursor_Hand);
                             if (ImGui::IsMouseClicked(ImGuiMouseButton_Left)) {
-                                ImGui::SetClipboardText(m.body.substr(a, b2 - a).c_str());
+                                // Offsets can outlive a thread change (an older message inserted above shifts
+                                // indices): never let substr throw std::out_of_range from the render loop.
+                                if (a >= 0 && static_cast<std::size_t>(a) <= m.body.size())
+                                    ImGui::SetClipboardText(m.body.substr(a, b2 - a).c_str());
                                 anyMsgClicked = true;
                             }
                         }