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/base.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import customtkinter as ctk
+
+if TYPE_CHECKING:
+    from gui.app import ArmaModManagerApp
+
+
+class BaseView(ctk.CTkFrame):
+    """
+    Common base for all view panels.
+
+    Each view is a CTkFrame owned by the app's content area. The app creates
+    view instances once and caches them; it calls refresh() on each navigation
+    so views can update their dynamic content without rebuilding the whole frame.
+
+    Subclasses must implement build() and may override refresh().
+    """
+
+    def __init__(self, parent: ctk.CTkFrame, app: ArmaModManagerApp) -> None:
+        super().__init__(parent, fg_color="transparent")
+        self.app = app
+        self.build()
+
+    def build(self) -> None:
+        """Construct all child widgets. Called once from __init__."""
+        raise NotImplementedError
+
+    def refresh(self) -> None:
+        """Re-query data and update dynamic widgets. Called on every navigation."""