Feature List

## Required
- [x] Input numbers via UI buttons
- [x] Provide basic operations
- [x] Add
- [x] Multiply
- [x] Divide
- [x] Subtract
- [x] Compute operation (equals)
- [x] Allow a decimal values
- [x] Allow numbers to be cleared
- [x] Clear all (C)
- [x] Clear entry (CE)
- [x] Backspace

## Optional -- nice to have
- [ ] Keyboard input (numpad, operations, enter, etc.)
- [ ] Better operation display (show previous operand and operation)
- [ ] Some kind of history?
- [ ] Repeat operations

## Unknowns/Risks/Issues
- Limited experience with Rust's UI setup; what exactly is involved in creating a widget?
- Illegal operations (mostly just divide by zero)
- Decimal places; can only have one per number, unless decimal is deleted

No files, tests, or specific entry points are named. Start by reviewing the Java Swing calculator UI and the unchecked optional items, then clarify which feature is wanted; done should mean the selected behavior works without breaking the completed calculator requirements.

Written by the indexing model from the issue text.