Update copyright string.

Author: trypsynth

app/constants.hpp

@@ -4,7 +4,7 @@
 
 inline const wxString APP_NAME = "Paperback";
 inline const wxString APP_VERSION = wxString::FromUTF8(PAPERBACK_VERSION_STRING);
-inline const wxString APP_COPYRIGHT = "Copyright (C) 2025 Quin Gillespie. All rights reserved.";
+inline const wxString APP_COPYRIGHT = "Copyright (C) 2025-2026 Quin Gillespie. All rights reserved.";
 inline const wxString APP_WEBSITE = "https://github.com/trypsynth/paperback";
 inline constexpr int POSITION_SAVE_THROTTLE_MS = 1000;
 inline constexpr int STATUS_UPDATE_THROTTLE_MS = 100;