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>

src/Marathon.UI/Pages/Export/ExportHub.razor

@@ -0,0 +1,58 @@
+@*
+    ExportHub — a top-level home for the Excel export so it is no longer gated
+    behind opening a single event's detail page. Hosts the shared ExportDialog
+    (date range + kind) which is not event-specific.
+*@
+
+@page "/export"
+@inject IStringLocalizer<SharedResource> L
+@inject IDialogService Dialog
+@inject ISnackbar Snackbar
+
+<PageTitle>@L["App.Title"] · @L["Export.Title"]</PageTitle>
+
+<section class="m-shell">
+    <header class="m-rise m-rise-1" style="display: grid; gap: var(--m-space-3); max-width: 880px;">
+        <span class="m-kicker">@L["Export.Kicker"]</span>
+        <h1 class="m-display" style="font-size: clamp(2rem, 4vw, 3rem);">@L["Export.Title"]</h1>
+        <p style="color: var(--m-c-ink-soft); max-width: 60ch;">@L["Export.Hub.Lede"]</p>
+    </header>
+
+    <hr class="m-rule--double" />
+
+    <div class="m-rise m-rise-2" style="display: grid; gap: var(--m-space-3);">
+        <div>
+            <button type="button" class="m-chip" @onclick="OpenExportDialog" data-test="export-hub-open">
+                @L["Export.Hub.Action"] →
+            </button>
+        </div>
+        <span class="m-mono" style="font-size: 0.75rem; color: var(--m-c-ink-soft); letter-spacing: 0.08em;">
+            @L["Export.Hub.FilenameHint"]
+        </span>
+    </div>
+</section>
+
+@code {
+    private async Task OpenExportDialog()
+    {
+        var parameters = new DialogParameters
+        {
+            ["InitialFrom"] = MoscowTime.Now.Date.AddDays(-7),
+            ["InitialTo"] = MoscowTime.Now.Date,
+        };
+
+        var options = new DialogOptions
+        {
+            CloseOnEscapeKey = true,
+            FullWidth = true,
+            MaxWidth = MaxWidth.Small,
+        };
+
+        var reference = await Dialog.ShowAsync<ExportDialog>(L["Export.Title"], parameters, options);
+        var result = await reference.Result;
+        if (result is { Canceled: false, Data: string path })
+        {
+            Snackbar.Add(string.Format(CultureInfo.CurrentCulture, L["Export.Success"].Value, path), Severity.Success);
+        }
+    }
+}