Allow customization of mode switching shortcuts (Alt+M vs Shift+Tab)

## Summary

Currently, Claude Code uses different keyboard shortcuts for mode switching on different platforms:

• Mac: Shift+Tab
• Windows: Alt+M (as workaround, see #3390)

This inconsistency creates a poor user experience when switching between platforms.

## Request

Please add the ability to customize mode switching shortcuts in Claude Code settings, or standardize the shortcut
across all platforms.

## Use Case

As a user who works across multiple platforms (Mac, Windows, Unix), I would like to:

1. Use the same keyboard shortcut for mode switching regardless of platform
2. Customize the shortcut to match my workflow preferences
3. Have consistent muscle memory across environments

## References

• Related issue: #3390 (Windows Shift+Tab not working)
• Related issue: #3368