feat: functional gridfinity calculator app

docs/PRD.md

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+# PRD - Gridfinity Calculator
+
+## 1. Overview
+The Gridfinity Calculator is a web tool designed to help makers plan their Gridfinity layouts. It calculates the number of baseplates that fit in a specific container and the maximum bin height allowed.
+
+## 2. Target Audience
+- 3D printing enthusiasts using the Gridfinity system.
+- Workshop organizers and hobbyists.
+
+## 3. Key Features
+- **Dimension Input:** Width, Length (Depth), and Height of the target drawer/container.
+- **Unit Toggle:** Support for Inches (decimal/fractional) and Millimeters.
+- **Spec Overrides:** Modify standard 42mm grid and 7mm height unit.
+- **Dynamic Results:**
+  - Total full grid squares (e.g., "5x4 grids").
+  - Fit percentage/fractional grids (e.g., "5.2 x 4.1 grids").
+  - Vertical Unit Height (U) for bins (e.g., "6U").
+  - Remaining space (gap) in mm/inches.
+
+## 4. Technical Constraints
+- Must be fast and lightweight.
+- Must work offline once loaded (single-page build).
+- Mobile-responsive design.
+
+## 5. Calculation Logic
+- `Grid Count = Floor(Drawer Dimension / Grid Spec)`
+- `Exact Fit = Drawer Dimension / Grid Spec`
+- `Bin Unit Height (U) = Floor((Drawer Height - Base Thickness) / Height Unit)`
+- *Default Base Thickness:* 4.8mm
+- *Default Grid Spec:* 42mm
+- *Default Height Unit:* 7mm