Dashboard: add Playlist Overview card and compact the sidebar

Restructure the dashboard into a two-column layout: a wide main column
holding the new Playlist Overview card, and a narrow (220px) right
sidebar with the compacted Quick Actions and Workflow Guide cards.

The Overview card answers "which playlist should I add media to, and
who will see it?" without opening each playlist. Per playlist it shows:
  - name, linked to that playlist's content manager
  - item count, total duration and current version
  - "N/M online" coverage, or a "no players" warning
  - one chip per assigned player, with an online/offline dot, linking
    to that player's manage page
  - an explicit empty state when no player is assigned

Players with no playlist are surfaced in a separate warning, since they
silently display nothing.

main.py builds this in a fixed number of queries: players are fetched
once and grouped by playlist_id, rather than calling
Playlist.players per row (which would be one query per playlist).

Layout notes:
- minmax(0, 1fr) on the main column so long names cannot widen the grid
- sidebar is sticky on tall screens and collapses below the main column
  under 900px, where the two small cards sit side by side
- sidebar cards used the global .card hover lift; suppressed for these

Also fixes a corrupted emoji (U+FFFD) in the System Status card.

Verified at 1600/1200/950/850/600px: no overflow, no button label
wrapping, and correct dark-mode colours.

app/blueprints/main.py

@@ -34,14 +34,44 @@ def dashboard():
         storage_mb = round(storage_mb / (1024 * 1024), 2)  # Convert to MB
     
     server_logs = get_recent_logs(20)
-    
+
+    # Per-playlist overview for the dashboard: which playlist holds what, and
+    # which players consume it. This answers "where should I add media / which
+    # playlist needs editing?" without opening each playlist.
+    #
+    # Players are fetched in a single query and grouped in Python rather than
+    # relying on Playlist.players per row, to avoid one query per playlist.
+    playlists = Playlist.query.order_by(Playlist.name).all()
+    players_by_playlist = {}
+    for player in Player.query.order_by(Player.name).all():
+        if player.playlist_id is not None:
+            players_by_playlist.setdefault(player.playlist_id, []).append(player)
+
+    playlist_overview = []
+    for playlist in playlists:
+        assigned = players_by_playlist.get(playlist.id, [])
+        playlist_overview.append({
+            'playlist': playlist,
+            'content_count': playlist.contents.count(),
+            'total_duration': playlist.total_duration,
+            'players': assigned,
+            'player_count': len(assigned),
+            'online_count': sum(1 for p in assigned if p.is_online),
+        })
+
+    # Players with no playlist produce no content on screen, so surface them.
+    unassigned_players = Player.query.filter(Player.playlist_id.is_(None))\
+        .order_by(Player.name).all()
+
     return render_template(
         'dashboard.html',
         total_players=total_players,
         total_playlists=total_playlists,
         total_content=total_content,
         storage_mb=storage_mb,
-        recent_logs=server_logs
+        recent_logs=server_logs,
+        playlist_overview=playlist_overview,
+        unassigned_players=unassigned_players
     )