fix(apps): ambient unit preference so every page honours the account units

Most data-backed pages were registered without an explicit UnitPref, so their ViewModels fell back to
"units ?? UnitPref.Metric" and rendered km / Wh-per-km / per-km while the account (and the web) is
imperial -- e.g. the Statistics page showed 78 km and Wh/km. Only the list pages (which self-subscribe to
settings) and the two pages I wired by hand (Live Map, Climate) were correct.

Systemic fix (web useUnits() parity) instead of threading units through ~40 page ctors + shell registrations:
- New App.Core UnitPrefAmbient.Current (defaults to UnitPref.Metric).
- ShellWindow publishes it from the account preference at startup and on every settings change, before the
  page rebuild -- so a recreated page renders in the new units.
- Swap the 166 production "?? UnitPref.Metric" fallbacks (144 files: pages, projections, sections/panels/
  cards, dashboard widget VMs, FormattedValues, TsVehicleHeroCard) to "?? UnitPrefAmbient.Current". Explicit
  units still win where passed; only the omitted-units path changes. Test fallbacks are untouched, and no
  test mutates the ambient, so the 31537 tests keep their deterministic metric default.

Gates: full sln build 0 err, dotnet format clean, 31537 tests pass.
Verified via live screenshot: Statistics (never edited individually) now renders 48 mi and Wh/mi matching
http://localhost:3000. Known follow-up: a few hardcoded per-km labels (e.g. "Cost per km") still need the
unit label -- the values already convert.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: atulmgupta

apps/windows/TeslaSync.App.Core/Units/UnitPrefAmbient.cs

@@ -0,0 +1,19 @@
+namespace TeslaSync.App.Core.Units;
+
+/// <summary>
+/// Process-wide ambient display-unit preference. The web resolves units per render via
+/// <c>useUnits()</c>; the native shell mirrors that by publishing the account's resolved preference here
+/// so that any ViewModel, projection or component constructed WITHOUT an explicit <see cref="UnitPref"/>
+/// falls back to the user's choice instead of hard-coded metric. Unit-aware fallbacks therefore read
+/// <c>units ?? UnitPrefAmbient.Current</c> rather than <c>units ?? UnitPrefAmbient.Current</c>.
+///
+/// <para>Defaults to <see cref="UnitPref.Metric"/> (SI) until the shell publishes the resolved preference.
+/// Only the shell mutates it (at startup and whenever the settings change), so unit tests — which never
+/// touch the shell — keep the metric default and stay deterministic. A <see cref="UnitPref"/> is an
+/// immutable record reference, so reads/writes of <see cref="Current"/> are atomic and need no locking.</para>
+/// </summary>
+public static class UnitPrefAmbient
+{
+    /// <summary>The current ambient display-unit preference (never null; defaults to metric).</summary>
+    public static UnitPref Current { get; set; } = UnitPref.Metric;
+}

apps/windows/TeslaSync.App/Components/DataDisplay/FormattedValues.cs

@@ -61,7 +61,7 @@ public UnitPref? Pref
     }
 
     /// <summary>The effective preference, falling back to metric defaults.</summary>
-    protected UnitPref EffectivePref => Pref ?? UnitPref.Metric;
+    protected UnitPref EffectivePref => Pref ?? UnitPrefAmbient.Current;
 
     /// <summary>The effective per-call precision, or null when the default applies.</summary>
     protected int? EffectivePrecision => Precision >= 0 ? Precision : null;

apps/windows/TeslaSync.App/Components/Vehicles/TsVehicleHeroCard.cs

@@ -106,7 +106,7 @@ public void SetState(VehicleHeroState? state)
     private static void OnPrefChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) =>
         ((TsVehicleHeroCard)d).Rebuild();
 
-    private UnitPref EffectivePref => Pref ?? UnitPref.Metric;
+    private UnitPref EffectivePref => Pref ?? UnitPrefAmbient.Current;
 
     private void BuildHeader()
     {

apps/windows/TeslaSync.App/Shell/ShellWindow.xaml.cs

@@ -14,6 +14,7 @@
 using TeslaSync.App.Core.Navigation;
 using TeslaSync.App.Core.Settings;
 using TeslaSync.App.Core.Theme;
+using TeslaSync.App.Core.Units;
 using TeslaSync.App.Notifications;
 using TeslaSync.App.Platform.Lifecycle;
 using TeslaSync.App.Push;
@@ -1397,6 +1398,10 @@ private void ConfigureWindow()
         _appliedMode = startup.ColorModeId;
         _appliedUnits = startup.Units;
 
+        // Publish the ambient display-unit preference before any page is built so unit-aware ViewModels
+        // constructed without explicit units fall back to the account preference (web useUnits parity).
+        UnitPrefAmbient.Current = startup.ToUnitPref();
+
         // The startup palette above is the local default; the deferred local-settings load and the
         // backend /settings theme seed (web ThemeProvider parity) arrive as later Changed events. Clear the
         // first-apply guard now so those events rebuild the already-constructed page when the theme actually
@@ -1921,6 +1926,10 @@ private void ApplySettings(AppSettings settings)
         _appliedUnits = settings.Units;
         _firstThemeApply = false;
 
+        // Republish the ambient unit preference before the page rebuild below so the recreated page (and any
+        // unit-aware ViewModel that defaults to the ambient) renders in the new units.
+        UnitPrefAmbient.Current = settings.ToUnitPref();
+
         ApplyDensity(settings.Density);
         MaybeApplyStartupRoute(settings);
 

apps/windows/TeslaSync.App/dashboard-widgets/AnalyticsSummaryWidget/AnalyticsSummaryWidget.ViewModel.cs

@@ -143,7 +143,7 @@ public AnalyticsSummaryViewModel(
         _source = source;
         _localizer = localizer;
         _size = size;
-        _units = units ?? UnitPref.Metric;
+        _units = units ?? UnitPrefAmbient.Current;
         _currencySymbol = string.IsNullOrWhiteSpace(currencySymbol) ? "$" : currencySymbol;
         _clock = clock ?? (() => DateTimeOffset.Now);
         _display = AnalyticsSummaryProjection.Project(AnalyticsSummary.Empty, _size, _units, _currencySymbol, _localizer);

apps/windows/TeslaSync.App/dashboard-widgets/ChargeCostTrackerWidget/ChargeCostTrackerWidget.ViewModel.cs

@@ -145,7 +145,7 @@ public ChargeCostTrackerViewModel(
         _source = source;
         _localizer = localizer;
         _size = size;
-        _units = units ?? UnitPref.Metric;
+        _units = units ?? UnitPrefAmbient.Current;
         _settings = settings ?? ChargeCostTrackerSettings.Default;
         _clock = clock ?? (() => DateTimeOffset.Now);
         _display = BuildDisplay(Array.Empty<ChargeCostTrackerSession>());

apps/windows/TeslaSync.App/dashboard-widgets/ChargeStatusLiveWidget/ChargeStatusLiveWidget.ViewModel.cs

@@ -142,7 +142,7 @@ public ChargeStatusLiveViewModel(
         _source = source;
         _localizer = localizer;
         _size = size;
-        _units = units ?? UnitPref.Metric;
+        _units = units ?? UnitPrefAmbient.Current;
     }
 
     /// <inheritdoc />

apps/windows/TeslaSync.App/dashboard-widgets/ChargeStatusWidget/ChargeStatusWidget.ViewModel.cs

@@ -141,7 +141,7 @@ public ChargeStatusViewModel(
         _source = source;
         _localizer = localizer;
         _size = size;
-        _units = units ?? UnitPref.Metric;
+        _units = units ?? UnitPrefAmbient.Current;
     }
 
     /// <inheritdoc />

apps/windows/TeslaSync.App/dashboard-widgets/ClimateControlPanelWidget/ClimateControlPanelWidget.ViewModel.cs

@@ -143,7 +143,7 @@ public ClimateControlPanelViewModel(
         _source = source;
         _localizer = localizer;
         _size = size;
-        _units = units ?? UnitPref.Metric;
+        _units = units ?? UnitPrefAmbient.Current;
     }
 
     /// <inheritdoc />

apps/windows/TeslaSync.App/dashboard-widgets/ClimateHistoryWidget/ClimateHistoryWidget.ViewModel.cs

@@ -128,7 +128,7 @@ public ClimateHistoryViewModel(
         _source = source;
         _localizer = localizer;
         _size = size;
-        _units = units ?? UnitPref.Metric;
+        _units = units ?? UnitPrefAmbient.Current;
         _display = ClimateHistoryProjection.Project(Array.Empty<ClimateHistorySample>(), _size, _units, _localizer);
     }