Add Core Spotlight indexing for documents, headings, and snippets

[clintecker]

Problem

Quoin has its own library and quick-open search, but documents and headings are not indexed for system search.

Apple's Core Spotlight documentation describes creating a private on-device index so system search can find app content, and Core Spotlight can also make indexed content available to broader system experiences.

Expected behavior

Users should be able to find Quoin documents, headings, tags/front matter, and snippets from Spotlight, then open directly to the matching document or heading.

Suggested fix

Add a small indexing service that:

• Observes LibraryModel scans / FSEvents changes.
• Indexes markdown documents with title, path, front matter, headings, and plain-text snippets.
• Uses stable identifiers that can route back to MainWindow.open plus scrollTarget.
• Deletes stale Spotlight items when files move or disappear.

Keep the index local-only and document privacy behavior.