fix: diff_buffers clear-to-end when deleting wide graphemes

[MomentDerek]

fix #4870 #4717

• Fix the chat composer “phantom space” bug that appeared when backspacing CJK (and other double-width) characters after the composer got a uniform background in 43b63cc….
• Pull diff_buffers’s clear-to-end logic forward to iterate by display width, so wide graphemes are counted correctly when computing the trailing column.
• Keep modifier-aware detection so styled cells are still flushed, and add a regression test (diff_buffers_clear_to_end_starts_after_wide_char) that covers the CJK deletion scenario.

[github-actions]

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[MomentDerek]

I have read the CLA Document and I hereby sign the CLA