Fix emoji and CJK cursor display width

[ryanoneill]

Summary

• Add cursor_display_position() to InputFieldState and TextAreaState that returns terminal column width using unicode-width, correctly handling wide characters (emoji, CJK) that occupy 2 terminal columns
• Update all 6 cursor rendering sites (InputField, TextArea, Form, LogViewer, DataGrid, ChatView) to use display width instead of character count
• Keeps existing cursor_position() API unchanged (175+ test call sites)

Before: Text "A😀B" with cursor after emoji — cursor_position() returns 2, cursor renders 1 column too far left
After: cursor_display_position() returns 3 (A=1col + 😀=2cols), cursor renders at correct position

Test plan

• New unit tests for cursor_display_position() on both InputField and TextArea (ASCII, emoji, CJK, mixed, empty, multiline)
• Doc tests on both new methods showing difference from cursor_position()
• All existing tests pass (278 tests)
• cargo clippy --all-targets --all-features -- -D warnings clean

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.61%. Comparing base (087bee6) to head (d7ce4bb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #117   +/-   ##
=======================================
  Coverage   92.61%   92.61%           
=======================================
  Files         100      100           
  Lines        9530     9535    +5     
=======================================
+ Hits         8826     8831    +5     
  Misses        704      704           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.