docs: add readme

README.md

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+# Gridfinity Calculator
+
+A fast, lightweight web tool for planning [Gridfinity](https://gridfinity.xyz/) modular storage layouts. Calculate exactly how many grids fit in your drawer, determine the maximum bin height, and identify remaining gaps for a perfect fit.
+
+## Features
+
+- **Unit Conversion**: Seamlessly switch between Millimeters and Inches with automatic value conversion.
+- **Precision Calculation**: Accurate grid counts (including fractional totals) based on standard 42mm Gridfinity specifications.
+- **Height Planning**: Calculates maximum bin units (U) based on drawer depth and baseplate thickness.
+- **Spec Overrides**: Customize grid size, unit height, and base thickness for specialized requirements.
+- **Responsive Design**: Clean, modern interface built with Tailwind CSS.
+
+## Getting Started
+
+### Prerequisites
+
+- [Node.js](https://nodejs.org/) (Latest LTS recommended)
+- [pnpm](https://pnpm.io/) (Recommended package manager)
+
+### Installation
+
+```bash
+# Install dependencies
+pnpm install
+```
+
+### Development
+
+Start the development server with Hot Module Replacement (HMR):
+
+```bash
+pnpm run dev
+```
+
+### Testing
+
+Run unit tests for the calculation engine and utility functions:
+
+```bash
+# Run tests once
+pnpm run test
+```
+
+### Building
+
+Build the project for production. The output will be in the `dist/` directory:
+
+```bash
+pnpm run build
+```
+
+### Deployment
+
+This project is configured for deployment to **Firebase Hosting**:
+
+```bash
+# Deploy to Firebase (requires Firebase login)
+pnpm run deploy
+```
+
+## Tech Stack
+
+- **Build Tool**: [Vite](https://vitejs.dev/)
+- **Language**: [TypeScript](https://www.typescriptlang.org/)
+- **Styling**: [Tailwind CSS](https://tailwindcss.com/)
+- **Testing**: [Vitest](https://vitest.dev/)
+- **Hosting**: [Firebase](https://firebase.google.com/)
+
+---
+
+*Based on the Gridfinity modular storage system by Zach Freedman.*