feat(scoreboard): units 7+8 — the layer's properties panel (facts, colours, plate, size)

Selecting a Scoreboard layer now shows its settings, using the app's own widgets only:
- eight fact checkboxes (subscribers, watch hours, Super Chats, new members, milestones, last member,
  last Super Chat, last gift)
- text colour and plate colour via YtComboBox over BorderColorPalette (the app's one palette —
  TASK 63's rule: a picker IS the app's drop-down, never a hand-built menu)
- plate on/off, plate opacity, text size 14-24

Every change pushes through the presenter (SetStyle/SetTiles) and repaints the strip, so the preview
and the output follow immediately. One look per layout, stored on the view-model: the scoreboard is a
single billboard, not eight independent widgets.

Guard: ScoreboardPropertiesTests asserts the panel is gated on SelectedElement.IsScoreboard, binds
all eight facts and the four look controls, uses YtComboBox/YtCheckBox only, and that the VM surface
exists for each name (a typo would otherwise bind to nothing in silence).

ViewModels/MainViewModel.Scoreboard.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
 using System.Windows.Media;
@@ -59,6 +60,76 @@ public partial class MainViewModel
     /// (unit 7) — kept here so the panel binds one object instead of six primitives.</summary>
     internal ScoreboardStyle ScoreboardStyleState => _scoreboardStyle;
 
+    // ─── Properties panel surface (per layout: one scoreboard look) ────────────
+
+    public string ScoreboardTextColor
+    {
+        get => _scoreboardStyle.TextColor;
+        set { _scoreboardStyle = _scoreboardStyle with { TextColor = value }; PushStyle(); }
+    }
+
+    public bool ScoreboardBackgroundEnabled
+    {
+        get => _scoreboardStyle.BackgroundEnabled;
+        set { _scoreboardStyle = _scoreboardStyle with { BackgroundEnabled = value }; PushStyle(); }
+    }
+
+    public string ScoreboardBackgroundColor
+    {
+        get => _scoreboardStyle.BackgroundColor;
+        set { _scoreboardStyle = _scoreboardStyle with { BackgroundColor = value }; PushStyle(); }
+    }
+
+    public double ScoreboardBackgroundOpacity
+    {
+        get => _scoreboardStyle.BackgroundOpacity;
+        set
+        {
+            _scoreboardStyle = _scoreboardStyle with { BackgroundOpacity = Math.Clamp(value, 0, 1) };
+            PushStyle();
+        }
+    }
+
+    public double ScoreboardFontSize
+    {
+        get => _scoreboardStyle.FontSize;
+        set
+        {
+            _scoreboardStyle = _scoreboardStyle with
+            {
+                FontSize = Math.Clamp(value, ScoreboardLayout.FloorFontSize, ScoreboardLayout.MaxFontSize),
+            };
+            PushStyle();
+        }
+    }
+
+    /// <summary>The colours offered for text and plate: the app's one palette.</summary>
+    public IReadOnlyList<NamedColor> ScoreboardColorOptions => BorderColorPalette.All;
+
+    public bool ScoreboardShowSubscribers { get => _scoreboardTiles.Subscribers; set => SetTile(t => t.Subscribers = value); }
+    public bool ScoreboardShowWatchHours { get => _scoreboardTiles.WatchHours; set => SetTile(t => t.WatchHours = value); }
+    public bool ScoreboardShowSuperChats { get => _scoreboardTiles.SuperChats; set => SetTile(t => t.SuperChats = value); }
+    public bool ScoreboardShowNewMembers { get => _scoreboardTiles.NewMembers; set => SetTile(t => t.NewMembers = value); }
+    public bool ScoreboardShowMilestones { get => _scoreboardTiles.Milestones; set => SetTile(t => t.Milestones = value); }
+    public bool ScoreboardShowLastMember { get => _scoreboardTiles.LastMember; set => SetTile(t => t.LastMember = value); }
+    public bool ScoreboardShowLastSuperChat { get => _scoreboardTiles.LastSuperChat; set => SetTile(t => t.LastSuperChat = value); }
+    public bool ScoreboardShowLastGift { get => _scoreboardTiles.LastGift; set => SetTile(t => t.LastGift = value); }
+
+    private void SetTile(Action<ScoreboardTiles> change)
+    {
+        change(_scoreboardTiles);
+        _scoreboardPresenter?.SetTiles(_scoreboardTiles);
+        RefreshScoreboardPreviews();
+        OnPropertyChanged(nameof(ScoreboardTilesState));
+    }
+
+    private void PushStyle()
+    {
+        _scoreboardPresenter?.SetStyle(_scoreboardStyle);
+        RefreshScoreboardPreviews();
+        OnPropertyChanged(nameof(ScoreboardStyleState));
+    }
+
     /// <summary>Publishes the strip to every scoreboard layer's preview element. The pump thread
     /// must never touch a WriteableBitmap, so this runs on the UI thread only.</summary>
     private void RefreshScoreboardPreviews()