refactor(gantry): use the shared speed util

Main added src/frontend/utils/units.ts as the single home for speed
conversion, so the Gantry copy has to go. Deletes gantryUnits.ts and
the two hooks that wrapped it. Nothing used them.

Renames the setting to speedUnit with 'mph' | 'km/h' | 'auto' values,
matching Battle, so it passes straight into resolveSpeedUnit with no
mapping in between.

Auto now falls back to mph when it cannot read iRacing's setting. That
is what the shared util does. It used to fall back to km/h.

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

Author: 23Kev

src/frontend/components/Gantry/hooks/gantryUnits.spec.ts

@@ -16,15 +16,25 @@ import { SettingDivider } from '../components/SettingDivider';
 import { SettingNumberRow } from '../components/SettingNumberRow';
 import { SettingSelectRow } from '../components/SettingSelectRow';
 import {
-  resolveGantryUnits,
-  speedFromDisplay,
-  speedMaxToDisplay,
-  speedMinToDisplay,
-  speedToDisplay,
-} from '../../Gantry/hooks/gantryUnits';
+  kphFromSpeed,
+  resolveSpeedUnit,
+  speedFromKph,
+  type SpeedUnit,
+} from '@irdashies/utils/units';
 
 const SETTING_ID = 'gantry';
 
+// Thresholds are stored in km/h; only the inputs convert. Bounds round inward
+// so a converted bound always lands back inside the stored range.
+const toDisplay = (kph: number, unit: SpeedUnit) =>
+  Math.round(speedFromKph(kph, unit));
+const fromDisplay = (value: number, unit: SpeedUnit) =>
+  Math.round(kphFromSpeed(value, unit));
+const minToDisplay = (kph: number, unit: SpeedUnit) =>
+  Math.ceil(speedFromKph(kph, unit));
+const maxToDisplay = (kph: number, unit: SpeedUnit) =>
+  Math.floor(speedFromKph(kph, unit));
+
 const defaultConfig = getWidgetDefaultConfig('gantry');
 
 type ThresholdKey =
@@ -153,10 +163,10 @@ export const GantrySettings = memo(() => {
   if (!currentDashboard) return <>Loading...</>;
 
   const config = settings.config;
-  const unitSetting = config.units ?? 'auto';
-  const { isMetric, speedUnit } = resolveGantryUnits(unitSetting, displayUnits);
+  const unitSetting = config.speedUnit ?? 'auto';
+  const speedUnit = resolveSpeedUnit(unitSetting, displayUnits);
   // This window has no TelemetryProvider, so Auto cannot read iRacing's setting
-  // here and falls back to km/h.
+  // here and falls back to the shared default.
   const autoUnresolved = unitSetting === 'auto' && displayUnits === undefined;
 
   return (
@@ -218,20 +228,20 @@ export const GantrySettings = memo(() => {
 
                 <SettingDivider />
 
-                <SettingButtonGroupRow<GantryConfig['units']>
+                <SettingButtonGroupRow<GantryConfig['speedUnit']>
                   title="Speed Units"
                   description={`Units for the speed settings on the Incidents tab. Values are always saved in km/h, so switching units never changes how incidents are detected.${
                     autoUnresolved
-                      ? " Auto follows iRacing's own unit setting, which this window cannot read, so it shows km/h here. Pick km/h or mph to choose explicitly."
+                      ? ` Auto follows iRacing's own unit setting, which this window cannot read, so it shows ${speedUnit} here. Pick km/h or mph to choose explicitly.`
                       : ''
                   }`}
                   value={unitSetting}
                   options={[
                     { label: 'Auto', value: 'auto' },
-                    { label: 'km/h', value: 'Metric' },
-                    { label: 'mph', value: 'Imperial' },
+                    { label: 'km/h', value: 'km/h' },
+                    { label: 'mph', value: 'mph' },
                   ]}
-                  onChange={(v) => handleConfigChange({ units: v })}
+                  onChange={(v) => handleConfigChange({ speedUnit: v })}
                 />
 
                 <SettingSelectRow
@@ -264,24 +274,24 @@ export const GantrySettings = memo(() => {
                       description={field.description}
                       value={
                         field.isSpeed
-                          ? speedToDisplay(config[field.key], isMetric)
+                          ? toDisplay(config[field.key], speedUnit)
                           : config[field.key]
                       }
                       min={
                         field.isSpeed
-                          ? speedMinToDisplay(field.min, isMetric)
+                          ? minToDisplay(field.min, speedUnit)
                           : field.min
                       }
                       max={
                         field.isSpeed
-                          ? speedMaxToDisplay(field.max, isMetric)
+                          ? maxToDisplay(field.max, speedUnit)
                           : field.max
                       }
                       step={1}
                       onChange={(v) =>
                         handleConfigChange({
                           [field.key]: field.isSpeed
-                            ? speedFromDisplay(v, isMetric)
+                            ? fromDisplay(v, speedUnit)
                             : v,
                         } as Partial<GantryConfig>)
                       }

src/frontend/components/Gantry/hooks/gantryUnits.ts

@@ -1420,7 +1420,7 @@ export const defaultDashboard: {
         height: 1080,
       },
       config: {
-        units: 'auto',
+        speedUnit: 'auto',
         slowSpeedThreshold: 15,
         slowFrameThreshold: 10,
         suddenStopFromSpeed: 80,

src/frontend/components/Gantry/hooks/useGantrySettings.ts

@@ -656,7 +656,7 @@ export type SessionRetention = 'all' | 5 | 10 | 20;
 
 export interface GantryConfig {
   /** Display units for speed values. Stored thresholds stay in km/h. */
-  units: 'auto' | 'Metric' | 'Imperial';
+  speedUnit: 'mph' | 'km/h' | 'auto';
   // Incident detection thresholds
   slowSpeedThreshold: number;
   slowFrameThreshold: number;