Add @witqq/spreadsheet to Spreadsheet section

[witqq]

@witqq/spreadsheet — A canvas-based spreadsheet engine with zero dependencies.

Why it's awesome

• Renders 100K+ rows at 60fps using Canvas 2D with zero external dependencies
• Framework-agnostic core with wrappers for React, Vue, and Angular
• Full spreadsheet features: formulas, sorting, filtering, clipboard, undo/redo, cell merge, frozen panes
• Real-time collaboration via OT engine
• TypeScript strict mode, ~36KB gzipped widget bundle

Links

• GitHub
• Live Demo
• npm

Added to Spreadsheet section.