feat: functional gridfinity calculator app

memory-bank/productContext.md

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+# Product Context
+
+## Why this project exists?
+Gridfinity is a popular modular storage system. While the standard is well-defined (42x42x7mm), users often need to:
+1. Figure out how many baseplates fit in a drawer.
+2. Determine the maximum height of bins for a specific drawer depth.
+3. Scale the system slightly to perfectly fill a drawer that doesn't fit a whole number of 42mm grids.
+
+## User Experience
+- **Input-First:** Users should immediately see where to enter their dimensions.
+- **Instant Feedback:** Results should update as they type.
+- **Flexibility:** Toggle between mm and inches easily.
+- **Precision:** Clear output showing "Full Grids" and "Vertical Units".
+
+## Success Criteria
+- Accurately converts between units.
+- Correctly calculates bin height while accounting for baseplate/bin-bottom thickness.
+- Provides clear instructions for "fractional" fits (e.g., if a drawer is 100mm, it fits 2.38 grids).