fix(webui): embedded backup-center review fixes

- BackupPage task-refresh run-guard is instance-relative now (window.backupPage
  OR window.overviewBackupPage), so the embedded center's own auto-refresh works
  instead of relying on OverviewManager's overlapping coverage.
- _ensureBackupAssets no longer memoizes a rejected promise — a transient
  script-load failure no longer bricks the backup center until reload; the next
  open retries.
- spaClean.loadScript removes the failed <script> element on error so the
  getElementById dedupe can't make a retry resolve without re-fetching.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -417,7 +417,11 @@ class OverviewManager {
       '/core/backup-card/js/backup-app-card.js',
     ];
     const load = (url) => (window.spaClean && window.spaClean.loadScript) ? window.spaClean.loadScript(url) : Promise.resolve();
-    this._backupAssets = scripts.reduce((p, url) => p.then(() => load(url)), Promise.resolve());
+    const chain = scripts.reduce((p, url) => p.then(() => load(url)), Promise.resolve());
+    // Don't memoize a rejection — a transient script-load failure would otherwise
+    // brick the backup center for the whole session. Clear the memo on failure so
+    // the next open retries.
+    this._backupAssets = chain.catch((e) => { this._backupAssets = null; throw e; });
     return this._backupAssets;
   }
 

containers/libreportal/frontend/components/backup/core/js/backup-page.js

@@ -79,7 +79,11 @@ class BackupPage {
             match: (d) => ['backup', 'restore', 'delete', 'delete_all'].includes(d.action)
                 || /^libreportal\s+(backup|restore)\b/.test((d.task && d.task.command) || ''),
             run: () => {
-                if (window.backupPage === this && document.getElementById('backup-page')) {
+                // Works whether this is the standalone page (window.backupPage) or
+                // the instance embedded in the Overview Backups tab
+                // (window.overviewBackupPage) — the hard-coded global alone would
+                // make the embedded center's auto-refresh dead.
+                if ((window.backupPage === this || window.overviewBackupPage === this) && document.getElementById('backup-page')) {
                     return this.refreshAll().then(() => this.render());
                 }
             },

containers/libreportal/frontend/core/kernel/js/spa.js

@@ -544,7 +544,9 @@ class LibrePortalSPAClean {
       script.src = src;
       script.id = scriptId;
       script.onload = resolve;
-      script.onerror = reject;
+      // Remove the element on failure so the getElementById() dedupe above can't
+      // make a later retry resolve instantly without re-fetching.
+      script.onerror = (e) => { try { script.remove(); } catch (_) {} reject(e); };
       document.head.appendChild(script);
     });
   }