Add track checkboxes to the bottom list and fix its attribute editor

[mattdawkins]

• Bottom track list rows get the same checkbox as the side list (a color box when solo), toggling the track through updateCheckedId
• The inline attribute editor's ref sits in a v-for, so Vue collected it into an array and focus() was never called; the field never blurred and stayed open. It now takes focus
• Column headers line up with their values: the header row was padded 8px more than the rows, and ID is centered over its values
• Length columns are 64px wide instead of 100px, for the value, the editor and the header

🤖 Generated with Claude Code