M5: Blazor dashboard (MudBlazor + ApexCharts)

- MudBlazor theme (dark default) + responsive drawer/appbar layout + nav.
- DashboardService read model: KPIs with period-over-period deltas, category breakdown,
  "what cost more/less" difference view, monthly trends.
- Pages: Overview (KPI cards + DeltaChip + donut + difference table), Trends (range-select
  bar chart), Meters (list + source status), Import (load reference dataset + CSV dry-run
  preview), Admin (energy types, tariffs). Charts isolated into components to avoid the
  ApexCharts/MudBlazor Color/Format name clashes.
- ReferenceDataImporter: one-click load of all four sheets as a starter dataset (meters,
  tank, tariff history, category memberships) — bundled sample CSVs copied to app output.
- End-to-end render test: import creates meters + consumption; overview/meters/trends/
  import/admin pages all return 200 with KPI cards rendered.

92 tests green (56 Core + 36 integration).

Deferred to polish: dedicated PV & oil/consumable panels, meter-detail page, full admin
CRUD, prev-year trend overlay.

Claude-Session: https://claude.ai/code/session_01WujdMtMJPbxDpDnMeK22rr

src/App/Components/Shared/TrendChart.razor

@@ -0,0 +1,31 @@
+@using ApexCharts
+@using System.Globalization
+
+@if (Points is { Count: > 0 })
+{
+    <ApexChart TItem="TrendPoint" Options="_options" Height="320">
+        <ApexPointSeries TItem="TrendPoint"
+                         Items="Points"
+                         SeriesType="SeriesType.Bar"
+                         Name="Monthly cost"
+                         XValue="Label"
+                         YValue="p => (decimal)Math.Round(p.Cost, 2)" />
+    </ApexChart>
+}
+else
+{
+    <MudText Typo="Typo.body2" Color="MudBlazor.Color.Secondary">No data in this range.</MudText>
+}
+
+@code {
+    [Parameter, EditorRequired]
+    public IReadOnlyList<TrendPoint> Points { get; set; } = [];
+
+    private readonly ApexChartOptions<TrendPoint> _options = new()
+    {
+        Theme = new Theme { Mode = Mode.Dark },
+        DataLabels = new DataLabels { Enabled = false },
+    };
+
+    private static object Label(TrendPoint p) => p.Period.ToString("MMM yy", CultureInfo.InvariantCulture);
+}