feat(ypp): session report derived from YouTube — no ledger, money off by default (TASK 3·20)

The task asked for a `RewardEvents` table plus a backfill. That would have contradicted the published
promise that no viewer, member or donor information is retained after the session ends, and it would
have meant owning a ledger that YouTube already keeps.

So the report is DERIVED on demand instead:

- `Services/SuperChatEventService.cs` — `superChatEvents.list`, parsed for both event types (a sticker
  must not be dropped), authorised by the existing `auth/youtube` scope, so no re-consent.
- `Models/SessionRewardSummary.cs` — rolls events up for the session window with **per-currency totals
  that are never summed together** (a $5 and a €5 are not $10). Nothing is written down.
- The YPP drawer gains a **THIS SESSION** section: counts are neutral and always shown when there were
  events; money appears only when the creator asks for it. `Show monetary metrics` is a drawer toggle,
  persisted, **off by default** — a creator who never monetized is never shown a $0.00.

Also: money is labelled as estimates (removed/refunded events are excluded and totals can be restated
for weeks, not days).

Tests: `SessionRewardReportTests` — both reward types parse, the window makes it a session rather than
a channel report, currencies stay separate, an empty session shows nothing, a sticker-only session
still counts. `BackOnAirTests` moves into the serialised `MainViewModelLayoutPath` collection: it builds
a MainViewModel directly and was flaking against parallel tests that swap the static layout path.

FAST gate: 557 passed, 0 failed, 0 warnings.

Controls/PreviewPane.xaml

@@ -790,6 +790,35 @@
                                                 <TextBlock Text="These arrive with the extended analytics update — subscriber + upload + standing progress is live now."
                                                            Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="12" TextWrapping="Wrap"/>
 
+                                                <!-- TASK 3·20 — THIS SESSION. Counts are neutral and always
+                                                     shown when there were events; money appears only if the
+                                                     creator turned it on AND there is something to show, so a
+                                                     creator who never monetized never sees a zero. Derived from
+                                                     YouTube on demand; nothing is stored. -->
+                                                <StackPanel Visibility="{Binding Ypp.HasSessionRewards, Converter={StaticResource BoolToVis}}">
+                                                    <TextBlock Text="THIS SESSION" Style="{StaticResource YtLabel}" Margin="0,14,0,2"/>
+                                                    <TextBlock Text="{Binding Ypp.SessionCountsText}"
+                                                               Foreground="{StaticResource Brush.OnSurface}" FontSize="13"/>
+                                                    <ItemsControl ItemsSource="{Binding Ypp.SessionMoneyLines}"
+                                                                  Visibility="{Binding Ypp.HasSessionMoney, Converter={StaticResource BoolToVis}}">
+                                                        <ItemsControl.ItemTemplate>
+                                                            <DataTemplate>
+                                                                <TextBlock Text="{Binding}" Foreground="{StaticResource Brush.OnSurfaceVariant}"
+                                                                           FontFamily="{StaticResource FontFamily.Mono}" FontSize="12" Margin="0,2,0,0"/>
+                                                            </DataTemplate>
+                                                        </ItemsControl.ItemTemplate>
+                                                    </ItemsControl>
+                                                    <TextBlock Text="Estimates from YouTube — removed or refunded events are excluded and totals can change."
+                                                               Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11"
+                                                               TextWrapping="Wrap" Margin="0,4,0,0"
+                                                               Visibility="{Binding Ypp.HasSessionMoney, Converter={StaticResource BoolToVis}}"/>
+                                                </StackPanel>
+
+                                                <CheckBox Content="Show monetary metrics" Style="{StaticResource YtCheckBox}"
+                                                          FontSize="12" Margin="0,10,0,0"
+                                                          IsChecked="{Binding Ypp.ShowMonetaryMetrics, Mode=TwoWay}"
+                                                          ToolTip="Off by default. Counts still appear; amounts stay hidden until you ask for them."/>
+
                                                 <TextBlock Text="COMPLIANCE CHECKLIST" Style="{StaticResource YtLabel}" Margin="0,14,0,4"/>
                                                 <TextBlock Text="CHANNEL STANDING" Style="{StaticResource YtLabel}" Margin="0,0,0,2"/>
                                                 <TextBlock Text="{Binding Ypp.StandingDetailText}" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11"