Remove combined savings, fix Net Savings

src/BikeTracking.Api.Tests/Application/Dashboard/GetAdvancedDashboardServiceTests.cs

@@ -426,7 +426,8 @@ public async Task GetAdvancedDashboardService_WithExpensesInWindow_IncludesExpen
     }
 
     [Fact]
-    public async Task GetAdvancedDashboardService_WithExpenses_NetSavingsIsCombinedMinusExpenses()
+    public async Task
+        GetAdvancedDashboardService_WithFuelCostAndMileageRate_NetSavingsUsesFuelCostOnly()
     {
         using var dbContext = CreateDbContext();
         var rider = await CreateRiderAsync(dbContext, "NetSavings Rider");
@@ -437,6 +438,8 @@ public async Task GetAdvancedDashboardService_WithExpenses_NetSavingsIsCombinedM
                 RiderId = rider.UserId,
                 RideDateTimeLocal = DateTime.Now.AddMonths(-3),
                 Miles = 100m,
+                SnapshotAverageCarMpg = 20m,
+                GasPricePerGallon = 3m,
                 SnapshotMileageRateCents = 67m,
                 CreatedAtUtc = DateTime.UtcNow,
             }
@@ -458,10 +461,13 @@ public async Task GetAdvancedDashboardService_WithExpenses_NetSavingsIsCombinedM
         var service = new GetAdvancedDashboardService(dbContext, TimeProvider.System);
         var result = await service.GetAsync(rider.UserId);
 
-        // 100 miles × 67 = 6700 combined savings - 30 expenses = 6670 net
-        Assert.Equal(6700m, result.SavingsWindows.AllTime.CombinedSavings);
+        // 100 miles / 20 mpg × $3 = $15 fuel cost avoided - $30 expenses = -$15 net
+        // Mileage-rate savings still exists, but it does not feed net savings.
+        Assert.Equal(15m, result.SavingsWindows.AllTime.FuelCostAvoided);
+        Assert.Equal(6700m, result.SavingsWindows.AllTime.MileageRateSavings);
+        Assert.Equal(6715m, result.SavingsWindows.AllTime.CombinedSavings);
         Assert.Equal(30m, result.SavingsWindows.AllTime.TotalExpenses);
-        Assert.Equal(6670m, result.SavingsWindows.AllTime.NetSavings);
+        Assert.Equal(-15m, result.SavingsWindows.AllTime.NetSavings);
     }
 
     [Fact]

src/BikeTracking.Api/Application/Dashboard/GetAdvancedDashboardService.cs

@@ -253,15 +253,12 @@ private static AdvancedSavingsWindow BuildWindow(
             oilChangeSavings = crossings > 0 ? RoundTo2(crossings * oilChangePrice.Value) : 0m;
         }
 
-        // Net savings: gross savings + oil-change offset − expenses.
-        // Null only when all savings are unavailable and there are no expenses.
+        // Net savings: fuel-cost avoided plus oil-change offset minus expenses.
         decimal? netSavings = null;
-        bool hasSavingsData = combinedSavings.HasValue || oilChangeSavings.HasValue;
+        bool hasSavingsData = fuelCostAvoided.HasValue || oilChangeSavings.HasValue;
         if (hasSavingsData || totalExpenses > 0m)
         {
-            netSavings = RoundTo2(
-                (combinedSavings ?? 0m) + (oilChangeSavings ?? 0m) - totalExpenses
-            );
+            netSavings = RoundTo2((fuelCostAvoided ?? 0m) + (oilChangeSavings ?? 0m) - totalExpenses);
         }
 
         return new AdvancedSavingsWindow(

src/BikeTracking.Api/Contracts/AdvancedDashboardContracts.cs

@@ -88,8 +88,8 @@ public sealed record AdvancedSavingsWindow(
     /// </summary>
     decimal? OilChangeSavings,
     /// <summary>
-    /// Net financial position: (FuelCostAvoided ?? 0) + (MileageRateSavings ?? 0) + (OilChangeSavings ?? 0) − TotalExpenses.
-    /// Null only when all savings components are null and expenses are zero.
+    /// Net financial position: (FuelCostAvoided ?? 0) + (OilChangeSavings ?? 0) − TotalExpenses.
+    /// Null only when fuel/oil savings are unavailable and expenses are zero.
     /// Can be negative when expenses exceed savings.
     /// </summary>
     decimal? NetSavings

src/BikeTracking.Frontend/src/pages/advanced-dashboard/SavingsWindowsTable.tsx

@@ -59,9 +59,6 @@ function WindowRow({ window: w }: WindowRowProps) {
         ) : null}
       </td>
       <td className="savings-windows-cell">{formatCurrency(w.mileageRateSavings)}</td>
-      <td className="savings-windows-cell savings-windows-combined">
-        {formatCurrency(w.combinedSavings)}
-      </td>
       <td className="savings-windows-cell">{formatCurrency(w.totalExpenses)}</td>
       <td className="savings-windows-cell">{formatCurrency(w.oilChangeSavings)}</td>
       <td
@@ -96,7 +93,6 @@ export function SavingsWindowsTable({
             <th className="savings-windows-cell savings-windows-header">Gallons Saved</th>
             <th className="savings-windows-cell savings-windows-header">Fuel Cost Avoided</th>
             <th className="savings-windows-cell savings-windows-header">Mileage Rate</th>
-            <th className="savings-windows-cell savings-windows-header">Combined Savings</th>
             <th className="savings-windows-cell savings-windows-header">Expenses</th>
             <th className="savings-windows-cell savings-windows-header">Oil Change Savings</th>
             <th className="savings-windows-cell savings-windows-header">Net Savings</th>

src/BikeTracking.Frontend/src/pages/advanced-dashboard/advanced-dashboard-page.css

@@ -150,11 +150,6 @@
   color: var(--adv-ink);
 }
 
-.savings-windows-combined {
-  font-weight: 700;
-  color: #065f46;
-}
-
 .savings-windows-row:last-child .savings-windows-cell {
   border-bottom: none;
 }

src/BikeTracking.Frontend/src/services/advanced-dashboard-api.ts

@@ -23,8 +23,8 @@ export interface AdvancedSavingsWindow {
    */
   oilChangeSavings: number | null;
   /**
-   * Net financial position: combinedSavings + oilChangeSavings − totalExpenses.
-   * Null only when all savings are null and expenses are zero. Can be negative.
+   * Net financial position: fuelCostAvoided + oilChangeSavings − totalExpenses.
+   * Null only when fuel/oil savings are unavailable and expenses are zero. Can be negative.
    */
   netSavings: number | null;
 }