fix(dashboard): admin-UI cleanup sweep

Family-wide admin-UI cleanup pass (scadaproj admin_ui_cleanup.md) applied to the
Blazor dashboard. Behaviour is unchanged throughout — no @onclick, disabled,
binding, auth gate, or arm->confirm flow was touched.

Uncontrolled error text is now truncated at the render site. Fault messages,
Galaxy load errors, and browse-tree load failures were rendered in full into
fixed-width table cells, where a long exception string blows out the column.
Each site gets DashboardDisplay.Abbreviate plus a title attribute carrying the
untruncated text, so nothing becomes unreachable. Abbreviate is length-checked
rather than a bare range slice: `value[..n]` on a shorter string throws and
takes the whole page render down with it. The two detail views whose entire
purpose is to show one fault in full — SessionDetailsPage and GalaxyPage's
Last Error — are deliberately left untruncated.

Two classes referenced from markup had no definition anywhere in the sheet.
.browse-stale-banner was inert; .tree-load-status was a real visual defect —
loading and failed-to-load rows sit among .tree-row siblings and carry the same
leading .tree-toggle-empty spacer, but that spacer only takes its width as a
flex item, so without a flex container those rows lost their indent.

Confirm/cancel pairs in ConfirmDialog and the API-key create form are now
btn-groups with role="group" and an aria-label, replacing margin-spaced loose
buttons.

Removes a paragraph on GalaxyPage naming internal RPCs (DiscoverHierarchy,
GetLastDeployTime) — implementation detail with no meaning to a dashboard
operator.

Verified in a real browser, not bUnit: full build clean, 879/879 tests, and a
live gate against a running dashboard with a genuine ~250-char SqlClient
exception as the erroring row. Results per check, including the checks that
could NOT be exercised without an x86 worker, are recorded in
docs/plans/2026-08-11-dashboard-ui-sweeps.md.

That plan doc also records a correction: this app is NOT Bootstrap-free. The
sweep brief said it was, citing the scadaproj index; libman.json pins
bootstrap 5.3.3 and App.razor:7 links it ahead of the theme. The stale claim had
already cost this app one skipped family sweep (scadaproj#2, the /admin/secrets
modal), so that modal was live-gated here too and passes.

src/ZB.MOM.WW.MxGateway.Server/Dashboard/Components/DashboardDisplay.cs

@@ -36,6 +36,32 @@ public static class DashboardDisplay
         return string.IsNullOrWhiteSpace(value) ? "-" : value;
     }
 
+    /// <summary>
+    /// Formats a nullable text value for display, shortened to a maximum length.
+    /// </summary>
+    /// <remarks>
+    /// For table cells bound to text the gateway does not control — fault messages, COM
+    /// exception text, SQL errors. One multi-line exception otherwise makes a single row
+    /// several times taller than its neighbours. Call sites keep the full text reachable
+    /// on the element's <c>title</c> and on the row's detail page.
+    /// </remarks>
+    /// <param name="value">The text to format.</param>
+    /// <param name="maxLength">Maximum characters to render before the ellipsis.</param>
+    /// <returns>Formatted text, ellipsized when longer than <paramref name="maxLength"/>, or "-" if null or empty.</returns>
+    public static string Abbreviate(string? value, int maxLength = 80)
+    {
+        if (string.IsNullOrWhiteSpace(value))
+        {
+            return "-";
+        }
+
+        // Length-checked, never a bare range slice: a value shorter than maxLength
+        // would throw and take the whole page render down with it.
+        return value.Length <= maxLength
+            ? value
+            : string.Concat(value.AsSpan(0, maxLength).TrimEnd(), "…");
+    }
+
     /// <summary>
     /// Formats a long count value for display with thousands separator.
     /// </summary>