perf(ui): optimize terminal cell width measurement (#579)

[kazu728]

In measureSanitizedTextWidth (src/ui/lib/text.ts), non-ASCII text falls to string-width, which classifies every grapheme cluster via /^\p{RGI_Emoji}$/v. So plain Japanese pays emoji-detection cost too, and that v-flag regex is pathologically slow on Bun.

[greptile-apps]

PR author is not in the allowed authors list.

[kazu728]

Relying on implicit transitive dependencies can cause confusion, so I added get-east-asian-width as an explicit dependency. It was already included transitively via string-width.