Trends: anchor empty/loading/error states to the top of the chart slot

The no-data states previously floated or centred vertically (Spacer
sandwich), so the sheet rendered them halfway down the page while the
chart-with-data anchors to the top. All chartArea states now share one
top-aligned container that fills the space below the pickers — the
empty state sits exactly where the chart will render, and the layout no
longer jumps between states. 83 tests, Release build green.

FuelBoard/TrendsView.swift

@@ -147,32 +147,38 @@ struct TrendsView: View {
 
     @ViewBuilder
     private var chartArea: some View {
-        if isLoading {
-            Spacer()
-            ProgressView("Fetching price history…")
-            Spacer()
-        } else if loadFailed {
-            Spacer()
-            VStack(spacing: 10) {
-                Image(systemName: "wifi.exclamationmark")
-                    .font(.system(size: 32))
-                    .foregroundStyle(.secondary)
-                Text("Couldn't load price history")
-                    .font(.headline)
-                Button("Retry") { Task { await load() } }
-                    .buttonStyle(.bordered)
-            }
-            Spacer()
-        } else if !hasAnyData {
-            emptyState
-        } else if !hasEnoughData {
-            emptyState // single point — nothing to draw yet
-        } else {
-            VStack(spacing: 12) {
-                chart
-                legend
+        // Every state anchors to the TOP of the chart slot — the same spot the
+        // chart occupies when data exists. The empty/loading/error states must
+        // not float or centre in the sheet, or the layout jumps between states.
+        Group {
+            if isLoading {
+                VStack(spacing: 12) {
+                    ProgressView("Fetching price history…")
+                }
+                .padding(.top, 24)
+            } else if loadFailed {
+                VStack(spacing: 10) {
+                    Image(systemName: "wifi.exclamationmark")
+                        .font(.system(size: 32))
+                        .foregroundStyle(.secondary)
+                    Text("Couldn't load price history")
+                        .font(.headline)
+                    Button("Retry") { Task { await load() } }
+                        .buttonStyle(.bordered)
+                }
+                .padding(.top, 24)
+            } else if !hasAnyData {
+                emptyState
+            } else if !hasEnoughData {
+                emptyState // single point — nothing to draw yet
+            } else {
+                VStack(spacing: 12) {
+                    chart
+                    legend
+                }
             }
         }
+        .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .top)
     }
 
     private var emptyState: some View {