# V3-03 — Distance and speed units

**Phase** Foundations · **Depends on** V3-02 · **Size** S · **Status** Not started

## Goal
Imperial as well as metric, chosen once and applied everywhere.

## Context
Everything is hardcoded metric: `formatDistance` switches m/km at 1000, `formatSpeed`
prints km/h, elevation prints metres. Fine in Canada, useless to anyone in the US or UK.

Cheap, and the kind of thing that makes an app feel unfinished when missing.

## Design
A `UnitSystem` enum (`metric`, `imperial`) in `Config`, defaulting from the device locale
on first launch.

**Conversion belongs in formatting only.** Storage stays SI — metres, km/h, metres of
altitude — forever. Converting at the storage layer would corrupt every existing ride and
break the parity harness.

| Value | Metric | Imperial |
|---|---|---|
| Distance | m / km | ft / mi |
| Speed | km/h | mph |
| Elevation | m | ft |

## Implementation
1. `UnitSystem` in `Config`; default from `Platform.localeName`
2. Extend `ui/format.dart` — every formatter takes the unit system
3. Thread it through: record screen, trips list, trip detail, charts, map legend
4. Exports stay SI regardless. GPX is metres by specification; changing that breaks
   consumers.

## Acceptance criteria
- [ ] Switching units updates every screen immediately
- [ ] Stored values are unchanged — verified by exporting before and after
- [ ] GPX/GeoJSON output is byte-identical across the two settings
- [ ] First launch picks a sensible default from the locale

## Tests
- Formatter tests for both systems, including the m→km and ft→mi boundaries
- **A test asserting export output does not change with the unit setting**
- Widget test toggling units and checking a rendered label

## Risks
The obvious trap is converting too deep in the stack. Guard it with the export test.

## Out of scope
Temperature, pace (min/km) — pace is arguably right for running, revisit after V3-01.