fix(webui/overview): paint Backups header immediately, confine spinner to content

Switching to the fleet Backups tab blanked the whole pane to a bare, headerless
lpLoadingBox() while the (script-heavy) backup center loaded, so it read as the
entire section reloading with no indication of where you'd landed — then the
header + content appeared at once when the fetch resolved.

Render the shared .ov-tab-header (💾 Backups + description + divider) up front and
drop the spinner into an .ov-tab-body beneath it, matching the per-app detail
tabs' header-first idiom (and the recent Admin board fix). The header stays put
and only the content area swaps in once BackupPage mounts.

The Migrate tab already does this — mountMigrate() injects its header and the
sub-tab strip is static, so its spinner is already confined to the Restore/Peers
sub-panel — so no change was needed there.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: librelad <librelad@digitalangels.vip>

containers/libreportal/frontend/components/apps/overview/js/overview-manager.js

@@ -618,7 +618,13 @@ class OverviewManager {
       } catch (_) {}
       return;
     }
-    pane.innerHTML = lpLoadingBox('Loading backup center…');
+    // Paint the shared header immediately and confine the spinner to the body
+    // BENEATH the divider — switching to Backups shows the tab's title straight
+    // away and only the content area swaps in once the (script-heavy) backup
+    // center finishes loading, instead of blanking the whole pane to a bare
+    // headerless loader. Mirrors the per-app detail tabs' header-first idiom.
+    pane.innerHTML = this.renderHeader('backups')
+      + `<div class="ov-tab-body">${lpLoadingBox('Loading backup center…')}</div>`;
     this._loadBackupCenter(pane, sub);
   }