Every note can now have an optional emoji icon (frontmatter `icon` field).
The icon is displayed in the editor header, note list, search results,
and Quick Open. Includes command palette commands "Set Note Icon" and
"Remove Note Icon", plus full test coverage (Vitest + Playwright).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
