Use compatible checkbox marker glyphs

[barkure]

Summary

Fixes #747.

• Replace the checked checkbox/menu marker 🗹 (U+1F5F9 BALLOT BOX WITH BOLD CHECK) with ■ (U+25A0 BLACK SQUARE).
• Replace the unchecked checkbox marker ☐ (U+2610 BALLOT BOX) with □ (U+25A1 WHITE SQUARE).
• These replacement glyphs are from the long-standing Unicode Geometric Shapes block, so they should render reliably in common terminal fonts on Windows, Linux, and macOS.
• The previous unchecked marker rendered smaller than the checked marker in some terminals, so the new ■ / □ pair also improves visual consistency between checked and unchecked states.

Happy to use a different marker pair if maintainers prefer another option.

Test plan

• Built successfully with:

  cargo build -p edit --release

• Manually tested the resulting binary on macOS.