Add per-author highlight colors

[kylemcd]

authors can now have persistent local colors across document highlights and comment threads.

• generated Radix colors stay stable, with picker overrides, deletion, and a global off switch
• vault indexing backfills existing authors and handles file changes without rewriting Markdown
• editor, Reading view, tables, drafts, mobile, and pop-outs share live color updates
• failed settings writes roll back, while disabled colors restore the original yellow highlight
• validation: npm run check, npm run build, and dark-theme browser verification

Closes #67