feat(ui): promote Excel export to a top-level destination

- Add an /export hub page that hosts the existing (date-range, not event-specific)
  ExportDialog, so export is no longer reachable only by opening an event detail.
- Add an Export entry under the System nav section.

src/Marathon.UI/Components/NavBody.razor

@@ -53,6 +53,10 @@
     </NavLink>
 
     <div class="m-nav__group" style="margin-top: var(--m-space-5);">@L["Nav.Section.System"]</div>
+    <NavLink class="m-nav__link" href="export">
+        <MudIcon Icon="@Icons.Material.Outlined.FileDownload" Size="Size.Small" />
+        <span>@L["Nav.Export"]</span>
+    </NavLink>
     <NavLink class="m-nav__link" href="settings">
         <MudIcon Icon="@Icons.Material.Outlined.Tune" Size="Size.Small" />
         <span>@L["Nav.Settings"]</span>