fix(ui): invariant decimals + LocaleState test isolation + drawer offset + bundled Plotly

Cross-cutting polish that surfaced while Phase 8 was being implemented:

* Invariant-culture formatting on every decimal ToString("0.00" / "0.##")
  (OddsCell, OddsTimeline, SeverityBadge, AnomalyEvidence,
  Pages/Events/Detail) — previously the comma/dot decimal separator
  switched with the locale and broke tabular alignment + tests.

* LocaleState constructor no longer mutates process-wide ambient culture;
  apply only happens through Set(...). Stops parallel bUnit test runs
  leaking ru-RU into each other's threads.

* MainLayout: drawer-open state now offsets main content / footer / appbar
  by the drawer width (248px) so the sidebar no longer overlaps content.
  Mobile breakpoint (≤720px) keeps the original full-width layout.

* wwwroot/index.html (Marathon.UI RCL): switched from Plotly CDN to the
  bundled "_content/Plotly.Blazor/plotly-2.35.3.min.js" — works offline
  and matches the Plotly.Blazor 5.4.1 version pin.

* Marathon.Hosts.WpfBlazor/wwwroot/index.html: host-level page that
  BlazorWebView's HostPage attribute resolves to. Same Plotly bundle,
  no autostart="false" (BlazorWebView auto-starts).

src/Marathon.UI/wwwroot/index.html

@@ -38,8 +38,8 @@
         <a class="dismiss" style="float: right; cursor: pointer; padding: 0 8px;">×</a>
     </div>
 
-    <!-- Plotly.js for OddsTimeline charts (Phase 6). Pinned major version 2.x. -->
-    <script src="https://cdn.plot.ly/plotly-2.35.2.min.js" charset="utf-8"></script>
+    <!-- Plotly.js for OddsTimeline charts (Phase 6). Bundled by Plotly.Blazor 5.4.1. -->
+    <script src="_content/Plotly.Blazor/plotly-2.35.3.min.js" charset="utf-8"></script>
 
     <script src="_framework/blazor.webview.js" autostart="false"></script>
     <script src="_content/MudBlazor/MudBlazor.min.js"></script>