Fix CJK word navigation in the TUI composer (#16829)

Addresses #16584

Problem: TUI word-wise cursor movement treated entire CJK runs as a
single word, so Option/Alt+Left and Right skipped too far when editing
East Asian text.

Solution: Use Unicode word-boundary segments within each non-whitespace
run so CJK text advances one segment at a time while preserving
separator and delete-word behavior, and add regression coverage for CJK
and mixed-script navigation.

Testing: Manually tested solution by pasting text that includes CJK
characters into the composer and confirmed that keyboard navigation
worked correctly (after confirming it didn't prior to the change).