feat(dashboard): revamp with at-a-glance strip, sparkline, modal, disk bar

Replace the four stat cards with a compact three-card strip: 7-day activity
with inline SVG sparkline (new ActivitySparkline), Running Now / Next Check
with per-source countdown list, and System with credential health + disk
usage bar.

- Add GET /downloads/activity-timeline — per-day completed/failed/files
  counts, pre-filled with zero buckets so the sparkline always has N points.
- Report filesystem-level usage via shutil.disk_usage in storage rollup,
  plus a live fallback in GET /settings so the capacity bar works before
  the first Celery rollup runs and for cached rows that predate the field.
- Extract Download Details into a reusable DownloadDetailsModal component
  and wire Recent Activity rows to open it (previously Downloads-page only).
- Compute next scheduled check per source from global schedule_interval;
  surface credential expiration/missing alerts scoped to platforms that
  actually have enabled sources.
- Two-speed polling (5s when downloads are active, 30s idle) with Page
  Visibility awareness so background tabs don't churn the API.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/tasks/maintenance.py

@@ -3,6 +3,7 @@
 import asyncio
 import logging
 import os
+import shutil
 from datetime import datetime, timedelta
 from pathlib import Path
 
@@ -38,6 +39,29 @@ def format_size(size_bytes: int) -> str:
     return f"{size_bytes:.1f} PB"
 
 
+def _disk_usage_stats(path: Path) -> dict:
+    """Return filesystem-level usage for the volume hosting `path`.
+
+    Uses shutil.disk_usage so we report the actual mount's free space (the
+    number that matters for capacity planning), not just what our downloads
+    subtree consumes.
+    """
+    try:
+        usage = shutil.disk_usage(path)
+        return {
+            "disk_total": usage.total,
+            "disk_used": usage.used,
+            "disk_free": usage.free,
+            "disk_total_formatted": format_size(usage.total),
+            "disk_used_formatted": format_size(usage.used),
+            "disk_free_formatted": format_size(usage.free),
+            "disk_percent_used": round(usage.used / usage.total * 100, 1) if usage.total else 0,
+        }
+    except OSError as e:
+        logger.warning(f"Could not read disk usage for {path}: {e}")
+        return {}
+
+
 def calculate_storage_stats() -> dict:
     """Calculate storage statistics for the downloads directory.
 
@@ -80,6 +104,7 @@ def calculate_storage_stats() -> dict:
         "total_size_formatted": format_size(total_size),
         "file_count": file_count,
         "calculated_at": datetime.utcnow().isoformat(),
+        **_disk_usage_stats(downloads_path),
     }