Flow: virtual "sum" meters that aggregate upstreams + demo chain

Adds the "sum meter" the user asked for: a meter that doesn't physically exist
but represents the sum of other meters in the flow view.

- FlowService: a Virtual-mode meter has no readings of its own; its flow value is
  the sum of its upstream meters, resolved in topological order (so "Summe Solar"
  = Solar 1 + Solar 2, and Grid + Summe Solar → House with the remainder as
  "Other" = export / battery / inverter losses).
- Meters editor: a hint when Mode = Virtual explaining the sum-meter behaviour.
- Reference data seeds the full demo chain: Solar 1 + Solar 2 → Summe Solar;
  Netz + Summe Solar → Haus → Auto + Other — so /energy/1 shows a multi-level
  Sankey out of the box.
- Fix: SankeyChart Unit was passed as the literal string "_graph.Unit" (missing
  @) so node labels read "_graph.Unit" instead of "kWh". Caught by screenshotting
  the live page.

Test: Virtual_sum_meter_aggregates_its_upstreams. 69 Core + 50 Integration =
119 green. Live-verified with a browser screenshot of the multi-level flow.

Claude-Session: https://claude.ai/code/session_01Lz2RqAsnQhetqWNoCDfexK

src/App/Components/Pages/EnergyView.razor

@@ -58,7 +58,7 @@ else
             <MudText Typo="Typo.caption" Color="Color.Secondary" Class="mb-2">
                 Where the top-level flow goes. Arrow thickness ∝ amount; "Other" is the unmetered remainder.
             </MudText>
-            <SankeyChart Nodes="_graph.Nodes" Links="_graph.Links" Unit="_graph.Unit" />
+            <SankeyChart Nodes="_graph.Nodes" Links="_graph.Links" Unit="@_graph.Unit" />
         }
         else
         {

src/App/Components/Pages/Meters.razor

@@ -78,6 +78,13 @@ else
                 <MudSelectItem T="MeterMode" Value="mode">@mode</MudSelectItem>
             }
         </MudSelect>
+        @if (_working.Mode == MeterMode.Virtual)
+        {
+            <MudAlert Severity="Severity.Info" Dense="true" Class="mb-2">
+                Virtual "sum" meter — it has no readings of its own. In the flow view it equals the sum of the
+                upstream meters you select below (e.g. Sum Solar = Solar 1 + Solar 2).
+            </MudAlert>
+        }
         <MudTextField @bind-Value="_working.Unit" Label="Unit" Required="true" Class="mb-2" />
         <MudNumericField T="double" @bind-Value="_working.InitialBaseline" Label="Initial register baseline" Class="mb-2" />
         <MudSelect T="string" @bind-Value="_working.Role" Label="PV role (optional)" Class="mb-2">