Fix cursor navigation in wrapped editor lines

[shellicar]

Summary

• Up/Down arrows navigate by terminal row within wrapped lines
• Cursor column wraps correctly within the current terminal row
• Fix coordinate mismatch between editor lines and terminal rows in sticky zone

Closes #135

Co-Authored-By: Claude noreply@anthropic.com

[bananabot9000]

LGTM. The wrap-aware cursor navigation logic in moveUp/moveDown is correct -- subRow calculation, cross-line boundary handling, prefix width accounting all check out. terminal.ts simplification (editorScreenLines - cursorRow - 1 and cursorCol % columns) is a nice cleanup.

Cleanup notes (non-blocking):

• Prompt string construction in ClaudeCli.ts is duplicated between up/down cases and likely duplicated from wherever the prompt is originally built. A getPromptWidth() helper would prevent drift.
• byteOffsetAtVisualWidth returns a code unit offset, not a byte offset. Minor naming inaccuracy.
• The duplicated variable naming (busyUp/busyDown, colsUp/colsDown) could be shared -- both cases use the same values.

Manually verified by author. Ship it.