Adopt string-width for terminal width calculations

[bananabot9000]

Problem

The CLI calculates how many terminal columns a string occupies using manual methods -- raw .length, hand-rolled ANSI regex stripping, and hardcoded emoji width assumptions. This produces incorrect results for:

• Emoji (👨‍👩‍👧 is 2 columns but multiple codepoints)
• CJK characters (2 columns each, treated as 1)
• Zero-width joiners, variation selectors, combining marks
• Non-standard ANSI sequences the current regex doesn't match

The result: cursor positioning, line wrapping, and sticky zone sizing are wrong whenever these characters appear in output or input. This is the root cause behind width-related rendering bugs (#133) and compounds the history/viewport issues (#134, #135).

Expected behaviour

A string like "hello 🍌 世界" should measure as 14 columns (5 + 1 + 2 + 1 + 2 + 2 + 1), not whatever .length returns (varies by JS engine, but wrong).

Proposed solution

Adopt string-width (v8.2.0). It handles all of the above correctly.

Dependency tree (3 transitive, all zero-dep leaves):

string-width@8.2.0
├── get-east-asian-width@1.5.0
└── strip-ansi@7.2.0
    └── ansi-regex@6.2.2

All MIT, all ESM, all pure JS, all sindresorhus. No native code.

Scope

Replace all manual width measurement with stringWidth() calls:

• renderer.ts -- cursor row wrap calculation
• terminal.ts -- editor line measurement, cursorLinesFromBottom, history line ANSI stripping
• StatusLineBuilder.ts -- screenLines() and potentially text()/emoji() methods
• buildPreviewLines() -- preview line wrapping

This is a refactor with no behaviour change (other than more accurate measurements). Should land before #134 and #135 so those fixes can rely on correct width calculations.