Show number fields with three decimals and snap dragging to the step

Number fields derived their display precision from `step`, so `step: 1`
(X/Y/Z, width, height, font size) rendered a stored 100.234 as "100" — and
because `onChange` parses the text, touching the field wrote the rounded
value back into the project. `formatNumber` now renders every number with
three decimals, independent of `step`, which is left to control the scrub
and arrow-key increment only.

Dragging now snaps to the increment's grid (`snapToStep`), so a scrub at
`step: 1` always lands on whole numbers even from a fractional start, while
Alt (x0.1) and the 0.1-step fields keep their precision. Typed values and
arrow keys are untouched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ui/index.ts

@@ -3,7 +3,7 @@ export { HStack, VStack, Spacer, Stack, type StackOptions } from './layout/Stack
 export { Theme, textStyle, type TextStyleToken } from './Theme';
 export { initUI, ui, type UIContext } from './UIContext';
 export { drawControlBox, drawGrid, type ControlBoxOptions, type GridOptions } from './draw';
-export { clamp, centreY, zoomFactor } from './math';
+export { clamp, centreY, formatNumber, NUMBER_DECIMALS, snapToStep, zoomFactor } from './math';
 export { Label } from './widgets/Label';
 export { Button, type ButtonOptions } from './widgets/Button';
 export { Dropdown } from './widgets/Dropdown';