feat(centralui): hide OffsetPager on a single page, opt-in — NotificationReport enabled (M10 residual)

DECISION: a pager whose only two controls are permanently disabled is noise, so
the conventional behaviour ships — the bar is hidden when the result set is a
single page. NotificationReport, which raised the finding, opts in.

RATIONALE for opt-in rather than default-on. OffsetPager is shared, and its
summary span ("Page N of Y · T total") doubles as the ONLY total-count readout
on ConfigurationAuditLog — auto-hiding there would silently delete the result
count for every query returning under a page, and the existing Playwright
fixture asserts "3 total" on exactly such a query. So the behaviour lives in the
shared component (reusable for the next consumer) behind HideWhenSinglePage,
default false, which keeps every current consumer byte-identical.

The guard fires only when single-page-ness is POSITIVELY established:
PageCount <= 1 AND Page <= 1 AND !HasNextPage. A null TotalCount means the host
cannot count, so the bar renders — the controls never vanish while a further
page still exists.

NotificationReport's own Playwright pagination test seeds 51 rows (2 pages) and
is unaffected. 5 new bUnit tests cover hide-on-single-page, hide-on-empty,
show-on-multi-page, show-when-TotalCount-unknown, and the default-off path.

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Notifications/NotificationReport.razor

@@ -204,7 +204,8 @@
                      HasNextPage="@(_notifications.Count >= _pageSize)"
                      TotalCount="_totalCount"
                      PageSize="_pageSize"
-                     Disabled="_loading" />
+                     Disabled="_loading"
+                     HideWhenSinglePage="true" />
     }
 </div>
 
@@ -326,7 +327,7 @@
                         @* Email bodies are plain text (design: BCC delivery, plain text).
                            Rendered as preformatted text — never as a MarkupString, which
                            would be an XSS vector. *@
-                        <pre class="border rounded bg-light p-2 mb-0"
+                        <pre class="border rounded bg-body-secondary p-2 mb-0"
                              style="max-height: 320px; overflow: auto; white-space: pre-wrap; word-break: break-word;">@_detail.Body</pre>
                     }
                 </div>