Implement core scientific calculator UX (multi-line expressions + keypad modes)

## Goal
Implement the baseline Desmos-style scientific calculator interaction model in Pretty-Calc.

## Scope
- Multi-line expression list (not single-entry only)
- Reuse previous answer token (`ans`)
- Three keypad modes:
- `main` (numbers/operators/roots/fractions/trig basics)
- `abc` (qwerty-style symbols/text helpers)
- `func` (advanced function entry)
- Decimal-to-fraction answer toggle (where rational approximation is available)
- Angle mode toggle (degrees/radians) in settings
- Calculator settings surface that can host future toggles

## Acceptance Criteria
- Users can enter and evaluate expressions across multiple lines.
- `ans` resolves to the previous evaluated result.
- Keypad mode switching is discoverable and preserves expression focus.
- Degree/radian mode changes affect trig evaluation correctly.
- Decimal/fraction representation can be toggled on supported outputs.
- Existing tests pass and new interaction tests are added for keypad mode + `ans` + angle mode.