Fix backslash key not mappable in controller settings

Summary

Fixes keyboard mapping for the backslash key (\) which was previously unmappable in controller button assignment.

Problem

When attempting to assign the backslash key to a controller button in input settings, the key press is silently ignored.

Root Cause

The mapping uses AvaKey.OemBackslash (European 102nd key) instead of AvaKey.OemPipe (US standard backslash key). Avalonia emits OemPipe across all platforms.

Solution

Changed line 144 in AvaloniaKeyboardMappingHelper.cs to map Key.BackSlash to AvaKey.OemPipe.

Testing

• Build passes with zero warnings/errors
• Code formatting passes all checks
• Manual testing: backslash key is now assignable in controller settings

Fixes #241