[Bug] UTF-8 string slicing panic with non-ASCII characters in code generation

Context

• I was implementing C++ header files (ConnectedComponent and DistanceTransform)
• The code/comments contained Chinese characters
• The UI showed "accept edits on (shift+tab to cycle)" when the crash occurred

Version

• Claude Code: 2.0.76

Expected Behavior

Claude Code should handle UTF-8 Chinese characters correctly without crashing.

Actual Behavior

The application panicked due to incorrect byte index calculation when slicing a string containing Chinese characters.