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/__init__.py

@@ -0,0 +1,24 @@
+"""
+gui — Arma Mod Manager UI package.
+
+Entry point:
+    from gui import run_app
+    run_app()
+"""
+from __future__ import annotations
+
+import customtkinter as ctk
+
+# Apply theme before any widget is created
+ctk.set_appearance_mode("dark")
+ctk.set_default_color_theme("blue")
+
+
+def run_app() -> None:
+    """Create and start the main window."""
+    from gui.app import ArmaModManagerApp
+    app = ArmaModManagerApp()
+    app.mainloop()
+
+
+__all__ = ["run_app"]