v1.8.71: fix Windows compatibility — guard Linux-only X11 code with platform check

- main.py: wrap _ensure_display_access() in sys.platform != "win32" check
- main.py: add FileNotFoundError to except in _find_active_xauthority()
- claude_setup.py: platform-aware error message for ssh-keygen not found
- CLAUDE.md: add mandatory cross-platform rules for all contributors

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

main.py

@@ -23,7 +23,7 @@ def _find_active_xauthority():
                 xauth = os.path.join(entry.path, ".Xauthority")
                 if os.path.isfile(xauth):
                     return xauth
-    except PermissionError:
+    except (PermissionError, FileNotFoundError):
         pass
     # 3. Check /root/.Xauthority
     root_xauth = "/root/.Xauthority"
@@ -105,7 +105,8 @@ from gui.app import App
 
 
 def main():
-    _ensure_display_access()
+    if sys.platform != "win32":
+        _ensure_display_access()
     app = App()
     app.mainloop()