fix(cli): draw grapheme clusters whole in the theme preview

The +list-themes interactive preview dropped the second emoji of ZWJ
clusters (e.g. woman + ZWJ + rocket) in the search box. Zig's libvaxis
enables Unicode grapheme mode 2027 for its session and sizes text per
grapheme cluster; the Rust preview did neither, so the terminal split
the cluster at the ZWJ and the cursor column was computed from summed
per-codepoint widths (4 instead of 2).
- The preview sends ?2027h on entry and ?2027l on exit like vaxis.
- tui::gwidth follows vaxis's unicode width rules per grapheme cluster
  (VS15/VS16, emoji presentation, regional-indicator pairs) using the
  ghostty-unicode emoji table; the preview's print path uses it.
+list-colors and +list-keybinds pretty output (also using gwidth) stays
byte-identical to /usr/bin/ghostty at widths 40/80/132.

Test Plan:
- cargo test -p ghostty-cli --test list_themes_pty
  zwj_emoji_cluster_is_drawn_whole_in_search_box
- cargo test --workspace --no-fail-fast

Cargo.lock

@@ -1436,6 +1436,7 @@ dependencies = [
  "ghostty-helpgen",
  "ghostty-input",
  "ghostty-os",
+ "ghostty-unicode",
  "ghostty-vt",
  "log",
  "ratatui",