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/api/settings.py

@@ -8,6 +8,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.models.setting import Setting, DEFAULT_SETTINGS, EXTENSION_API_KEY_SETTING, STORAGE_STATS_SETTING, generate_api_key
 from app.config import get_settings
+from app.services.gallery_dl import GalleryDLService
 
 bp = Blueprint("settings", __name__)
 
@@ -23,15 +24,24 @@ async def get_cached_storage_stats(session: AsyncSession) -> dict:
     )
     setting = result.scalar_one_or_none()
 
-    if setting and setting.value:
-        return setting.value
+    from app.tasks.maintenance import _disk_usage_stats
+    disk_path = Path(get_settings().download_path)
 
-    # No cached stats yet - return placeholder
+    if setting and setting.value:
+        cached = dict(setting.value)
+        # Older cached rollups predate the disk_* fields; top them up live so
+        # the capacity bar works without waiting for the next Celery run.
+        if "disk_percent_used" not in cached:
+            cached.update(_disk_usage_stats(disk_path))
+        return cached
+
+    # No cached stats yet - return placeholder plus live disk usage.
     return {
         "total_size": 0,
         "total_size_formatted": "Calculating...",
         "file_count": 0,
         "calculated_at": None,
+        **_disk_usage_stats(disk_path),
     }
 
 
@@ -178,16 +188,45 @@ async def get_gallery_dl_config():
     config_path = Path(config.config_path) / "gallery-dl.conf"
 
     if not config_path.exists():
-        return jsonify({"config": {}, "message": "No configuration file found"})
+        defaults = GalleryDLService()._get_default_config()
+        return jsonify({
+            "config": defaults,
+            "is_default": True,
+            "message": "No configuration file found — showing computed defaults. Save to persist.",
+        })
 
     try:
         with open(config_path) as f:
             gallery_dl_config = json.load(f)
-        return jsonify({"config": gallery_dl_config})
+        return jsonify({"config": gallery_dl_config, "is_default": False})
     except json.JSONDecodeError as e:
         return jsonify({"error": f"Invalid JSON in config file: {e}"}), 500
 
 
+@bp.route("/gallery-dl/reset", methods=["POST"])
+async def reset_gallery_dl_config():
+    """Write computed defaults to gallery-dl.conf, replacing any existing file.
+
+    Users can use this to recover after misconfiguring the conf via the
+    Settings view. The defaults come from GalleryDLService._get_default_config,
+    which includes per-platform sections (extractor.discord, extractor.patreon,
+    etc.) with sane gallery-dl-compatible values.
+    """
+    config = get_settings()
+    config_path = Path(config.config_path) / "gallery-dl.conf"
+    config_path.parent.mkdir(parents=True, exist_ok=True)
+
+    defaults = GalleryDLService()._get_default_config()
+    try:
+        with open(config_path, "w") as f:
+            json.dump(defaults, f, indent=4)
+        current_app.logger.info("Reset gallery-dl configuration to defaults")
+        return jsonify({"message": "Configuration reset to defaults", "config": defaults})
+    except Exception as e:
+        current_app.logger.error(f"Failed to write gallery-dl defaults: {e}")
+        return jsonify({"error": f"Failed to write defaults: {e}"}), 500
+
+
 @bp.route("/gallery-dl", methods=["PUT"])
 async def update_gallery_dl_config():
     """Update gallery-dl configuration."""