Add GUI desktop application

- Add gui/ package: CustomTkinter app with dashboard, mods, tools, logs,
  and settings views; first-run SetupWizard for config.json generation
- Add gui.py root entry point (calls gui.run_app())
- Add selection.json for GUI selection state persistence
- Add customtkinter to requirements.txt
- Fix link_mods.py minor issues surfaced during GUI integration
- Add modlist_html/Test_Preset_A.html and Test_Preset_B.html as example inputs
- Update README.md: add GUI prerequisites, gui.py script section, gui/ folder
  structure, customtkinter to prerequisites table
- Update CLAUDE.md: add python gui.py to common commands, document GUI package
  architecture and views

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui/views/__init__.py

@@ -0,0 +1,7 @@
+from gui.views.dashboard import DashboardView
+from gui.views.mods      import ModsView
+from gui.views.tools     import ToolsView
+from gui.views.logs      import LogsView
+from gui.views.settings  import SettingsView
+
+__all__ = ["DashboardView", "ModsView", "ToolsView", "LogsView", "SettingsView"]