Alt-arrow word navigation broken with Cyrillic and non-Latin characters

Description

When typing in the prompt input field using Cyrillic (and likely other non-Latin character sets), the Alt-arrow keyboard shortcut for word navigation does not work correctly.

Expected Behavior

Alt-Left/Alt-Right should jump to the beginning/end of the current/next word, consistent with standard text editor behavior.

Actual Behavior

When using Cyrillic or other non-Latin characters, Alt-arrow jumps over a large number of characters instead of moving word-by-word as expected.

Steps to Reproduce

1. Open Claude Code prompt input
2. Type text using Cyrillic characters (e.g., Russian: "это тестовый текст")
3. Press Alt-Left or Alt-Right arrow keys
4. Observe that the cursor skips over many characters instead of moving to word boundaries

Environment

• Character sets affected: Cyrillic (confirmed), likely other non-Latin encodings
• Feature: Prompt input field

Additional Context

This appears to be a word boundary detection issue where the text navigation logic may not correctly recognize word boundaries in non-ASCII character sets.