You're helping a first-time DIY painter figure out how many cans of paint to buy before heading to the hardware store.

Build a single-page app where the user enters:
- **Room dimensions**: wall width and height in feet (up to 4 walls)
- **Number of coats** (1 or 2)
- **Door/window cutouts**: how many doors and windows to subtract (standard sizes: door = 20 sq ft, window = 15 sq ft)
- **Paint can size**: choose 1-quart (100 sq ft) or 1-gallon (400 sq ft) coverage

The app instantly calculates total paintable square footage, total paint needed, and displays **how many cans to buy** (always rounded up). Show a clear summary card with the square footage breakdown and a friendly recommendation like "Buy 3 gallons" with a small note on any leftover coverage. Keep everything on one page with live recalculation as values change — no page reloads, no backend needed.