Build a single-page tool that helps a driver figure out how many fuel stops they'll need on a road trip and roughly how much gas will cost.

The user enters:
- **Total trip distance** (in miles or km, toggle between units)
- **Vehicle fuel efficiency** (mpg or L/100km, matching the unit toggle)
- **Tank size** (gallons or liters)
- **Current fuel price** (per gallon or per liter)
- **Starting tank level** (as a percentage, e.g. 75%)

The app instantly calculates and displays:
1. **Number of fuel stops needed** (assuming they refuel when the tank hits ~10% as a safety buffer)
2. **Total fuel cost** for the entire trip
3. **Approximate miles/km between each stop**

All results update live as the user types — no submit button needed. The layout should be clean and readable at a glance, like something you'd pull up on your phone before hitting the highway.