fix(prompts): truncate wide-character box titles by display width

[greymoth-jp]

What does this PR do?

box() measures the title and every line with fast-string-width, but it truncates an over-long title with title.slice(0, maxTitleLength - 3), which counts UTF-16 code units rather than display columns. maxTitleLength is a display-width budget, so for a wide-character (CJK/full-width) title the sliced result is still far too wide. The top border comes out ragged, and once getPaddingForLine returns a negative remainder, '─'.repeat(...) throws RangeError: Invalid count value.

Repro on main:

import { box } from '@clack/prompts';

// '这是一个非常长的标题' is 10 full-width chars = 20 display columns
box('message', '这是一个非常长的标题', { width: 0.2 });
// RangeError: Invalid count value: -6

The fix replaces the code-unit slice with a small truncateToWidth helper that accumulates characters by display width, the same way the rest of box() already measures. For ASCII titles it returns exactly what the old slice did, so existing snapshots are unchanged; only wide-character titles change behaviour (they now fit the box and the borders stay aligned).

No linked issue: this is a small self-contained bug fix.

Type of change

• Bug fix
• Feature
• Refactor (no behavior change)
• Documentation
• Performance improvement
• Tests
• Chore (dependencies, CI, tooling)

Checklist

• pnpm test passes (or targeted tests for my change)
• pnpm format has been run
• I have added/updated tests for my changes (if applicable)
• I have added a changeset

AI-generated code disclosure

• This PR includes AI-generated code

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🦋 Changeset detected

Latest commit: 2aee058

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@clack/prompts	Patch

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[43081j]

the ellipsis is constant so should be moved out of the function, and the length doesn't need to use stringWidth since we know it has no wide characters

[43081j]

this should really be in common.ts since it will be useful in other places too

[43081j]

Suggested change

	if (width + charWidth > target) {
	break;
	}
	result += char;
	width += charWidth;
	width += charWidth;
	if (width > target) {
	break;
	}
	result += char;

to avoid computing it twice