feat: add blockquote reference and hover-preview system

## Summary - Adds labeled blockquote references for `definition`, `theorem`, `lemma`, `algorithm`, and `problem` types - Reference links use a `#bqref-<id>` prefix and render as auto-numbered labels (e.g. "Definition 1.1") with optional hover-preview tooltips - Tooltip previews match blockquote background colors and inherit document fonts/text color - Mirrors the existing equation reference system architecture (`MarkdownProcessor` builds a mapping at parse time; a `useEffect` in `Notie.tsx` intercepts matching links in the DOM) - Adds `previewBlockquotes` config option (default `true`) to opt out of tooltips - Adds a demo page (`blockquote-reference.md`) and updates README ## Test plan - [ ] Add a labeled blockquote and a `[text](#bqref-<id>)` link — renders as "Type X.Y" with correct numbering - [ ] Hover over the reference link — tooltip shows blockquote content with matching background color and fonts - [ ] Click the link — navigates to the blockquote in the page - [ ] Set `previewBlockquotes: false` — renders plain link with no tooltip - [ ] Theorem + lemma references share a counter and number correctly - [ ] Invalid `#bqref-` id renders in red - [ ] `npm run build` and `npm run lint` pass 🤖 Generated with [Claude Code](https://claude.com/claude-code)