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

@@ -147,12 +147,16 @@ struct TrendsView: View {
 
     @ViewBuilder
     private var chartArea: some View {
+        // 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 {
-            Spacer()
+                VStack(spacing: 12) {
                     ProgressView("Fetching price history…")
-            Spacer()
+                }
+                .padding(.top, 24)
             } else if loadFailed {
-            Spacer()
                 VStack(spacing: 10) {
                     Image(systemName: "wifi.exclamationmark")
                         .font(.system(size: 32))
@@ -162,7 +166,7 @@ struct TrendsView: View {
                     Button("Retry") { Task { await load() } }
                         .buttonStyle(.bordered)
                 }
-            Spacer()
+                .padding(.top, 24)
             } else if !hasAnyData {
                 emptyState
             } else if !hasEnoughData {
@@ -174,6 +178,8 @@ struct TrendsView: View {
                 }
             }
         }
+        .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .top)
+    }
 
     private var emptyState: some View {
         VStack(spacing: 10) {