Add thumbnail strip to live progress monitor

Show small thumbnails of recently completed ArchiveResult content in the
progress header. The thumbnail strip appears below the stats bar and shows
the last 20 successfully archived items with embeddable content (screenshots,
favicons, DOM snapshots, etc.).

Features:
- API returns recent_thumbnails with embed paths for succeeded results
- Thumbnails display with plugin-specific icons as fallback
- New thumbnails animate in with a pop effect
- Clicking a thumbnail navigates to the snapshot admin page
- Horizontal scrollable strip with custom scrollbar styling

archivebox/core/views.py

@@ -565,6 +565,29 @@ def live_progress_view(request):
         archiveresults_succeeded = ArchiveResult.objects.filter(status=ArchiveResult.StatusChoices.SUCCEEDED).count()
         archiveresults_failed = ArchiveResult.objects.filter(status=ArchiveResult.StatusChoices.FAILED).count()
 
+        # Get recently completed ArchiveResults with thumbnails (last 20 succeeded results)
+        recent_thumbnails = []
+        recent_results = ArchiveResult.objects.filter(
+            status=ArchiveResult.StatusChoices.SUCCEEDED,
+        ).select_related('snapshot').order_by('-end_ts')[:20]
+
+        for ar in recent_results:
+            embed = ar.embed_path()
+            if embed:
+                # Only include results with embeddable image/media files
+                ext = embed.lower().split('.')[-1] if '.' in embed else ''
+                is_embeddable = ext in ('png', 'jpg', 'jpeg', 'gif', 'webp', 'svg', 'ico', 'pdf', 'html')
+                if is_embeddable or ar.plugin in ('screenshot', 'favicon', 'dom'):
+                    recent_thumbnails.append({
+                        'id': str(ar.id),
+                        'plugin': ar.plugin,
+                        'snapshot_id': str(ar.snapshot_id),
+                        'snapshot_url': ar.snapshot.url[:60] if ar.snapshot else '',
+                        'embed_path': embed,
+                        'archive_path': f'/archive/{ar.snapshot.timestamp}/{embed}' if ar.snapshot else '',
+                        'end_ts': ar.end_ts.isoformat() if ar.end_ts else None,
+                    })
+
         # Build hierarchical active crawls with nested snapshots and archive results
         from django.db.models import Prefetch
 
@@ -689,6 +712,7 @@ def live_progress_view(request):
             'archiveresults_succeeded': archiveresults_succeeded,
             'archiveresults_failed': archiveresults_failed,
             'active_crawls': active_crawls,
+            'recent_thumbnails': recent_thumbnails,
             'server_time': timezone.now().isoformat(),
         })
     except Exception as e:
@@ -708,6 +732,7 @@ def live_progress_view(request):
             'archiveresults_succeeded': 0,
             'archiveresults_failed': 0,
             'active_crawls': [],
+            'recent_thumbnails': [],
             'server_time': timezone.now().isoformat(),
         }, status=500)