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/Pages/Dashboard.razor

@@ -0,0 +1,102 @@
+@page "/"
+@rendermode InteractiveServer
+@inject DashboardService Dash
+
+<PageTitle>MeterVault — Overview</PageTitle>
+
+<MudText Typo="Typo.h4" Class="mb-4">Overview</MudText>
+
+@if (_summary is null)
+{
+    <MudProgressLinear Indeterminate="true" Color="Color.Primary" />
+}
+else
+{
+    <MudGrid>
+        <MudItem xs="12" sm="4">
+            <MudPaper Class="pa-4" Elevation="2">
+                <MudText Typo="Typo.overline" Color="Color.Secondary">This month</MudText>
+                <MudText Typo="Typo.h5">@Format.Euro(_summary.Month.Current)</MudText>
+                <DeltaChip Kpi="_summary.Month" />
+            </MudPaper>
+        </MudItem>
+        <MudItem xs="12" sm="4">
+            <MudPaper Class="pa-4" Elevation="2">
+                <MudText Typo="Typo.overline" Color="Color.Secondary">This year</MudText>
+                <MudText Typo="Typo.h5">@Format.Euro(_summary.Year.Current)</MudText>
+                <DeltaChip Kpi="_summary.Year" />
+            </MudPaper>
+        </MudItem>
+        <MudItem xs="12" sm="4">
+            <MudPaper Class="pa-4" Elevation="2">
+                <MudText Typo="Typo.overline" Color="Color.Secondary">Latest month with data</MudText>
+                <MudText Typo="Typo.h5">@Format.Euro(_summary.LatestMonthCost)</MudText>
+            </MudPaper>
+        </MudItem>
+
+        <MudItem xs="12" md="6">
+            <MudPaper Class="pa-4" Elevation="2" Style="height:100%">
+                <MudText Typo="Typo.h6" Class="mb-2">What costs most (this year)</MudText>
+                @if (_breakdown is { Count: > 0 })
+                {
+                    <CategoryDonut Slices="_breakdown" />
+                    <MudSimpleTable Dense="true" Hover="true" Class="mt-2">
+                        <tbody>
+                            @foreach (var slice in _breakdown)
+                            {
+                                <tr>
+                                    <td>@slice.Name</td>
+                                    <td style="text-align:right">@Format.Euro(slice.Cost)</td>
+                                </tr>
+                            }
+                        </tbody>
+                    </MudSimpleTable>
+                }
+                else
+                {
+                    <MudText Typo="Typo.body2" Color="Color.Secondary">No cost data yet — import a sheet or add tariffs.</MudText>
+                }
+            </MudPaper>
+        </MudItem>
+
+        <MudItem xs="12" md="6">
+            <MudPaper Class="pa-4" Elevation="2" Style="height:100%">
+                <MudText Typo="Typo.h6" Class="mb-2">What cost more / less (year vs last year)</MudText>
+                <MudSimpleTable Dense="true" Hover="true">
+                    <thead>
+                        <tr><th>Category</th><th style="text-align:right">Now</th><th style="text-align:right">Prev</th><th style="text-align:right">Δ</th></tr>
+                    </thead>
+                    <tbody>
+                        @foreach (var row in _difference)
+                        {
+                            <tr>
+                                <td>@row.Name</td>
+                                <td style="text-align:right">@Format.Euro(row.Current)</td>
+                                <td style="text-align:right">@Format.Euro(row.Previous)</td>
+                                <td style="text-align:right" class="@(row.Delta >= 0 ? "mv-up" : "mv-down")">
+                                    @Format.DirectionIcon(Math.Sign(row.Delta)) @Format.Euro(Math.Abs(row.Delta))
+                                </td>
+                            </tr>
+                        }
+                    </tbody>
+                </MudSimpleTable>
+            </MudPaper>
+        </MudItem>
+    </MudGrid>
+}
+
+@code {
+    private DashboardSummary? _summary;
+    private IReadOnlyList<CategorySlice> _breakdown = [];
+    private IReadOnlyList<DifferenceRow> _difference = [];
+
+    protected override async Task OnInitializedAsync()
+    {
+        var asOf = DateOnly.FromDateTime(DateTime.UtcNow);
+        _summary = await Dash.GetSummaryAsync(asOf);
+
+        var yearStart = new DateOnly(asOf.Year, 1, 1);
+        _breakdown = await Dash.GetCategoryBreakdownAsync(yearStart, asOf.AddMonths(1));
+        _difference = await Dash.GetCategoryDifferenceAsync(yearStart, yearStart.AddYears(-1), asOf.AddMonths(1));
+    }
+}