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/_io.py

@@ -0,0 +1,19 @@
+from __future__ import annotations
+
+import io
+import queue
+
+
+class _QueueWriter(io.TextIOBase):
+    """Redirect sys.stdout / sys.stderr into a Queue for the Logs panel."""
+
+    def __init__(self, q: queue.Queue[str]) -> None:
+        self._q = q
+
+    def write(self, text: str) -> int:  # type: ignore[override]
+        if text:
+            self._q.put(text)
+        return len(text)
+
+    def flush(self) -> None:
+        pass