UI: featured hero card, vivid status colors, themed Markdown render

Featured version
----------------
The highest installed version (or highest remote if none installed) now
gets a large hero card at the top of the window: 32px Proserve title,
oversized colored status pill, and a much bigger primary action button
(LANCER / INSTALLER 56px padding). All other versions move below into a
"AUTRES VERSIONS" section with a horizontal divider, displayed as the
existing compact rows.

MainViewModel exposes FeaturedVersion + OtherVersions instead of one
flat Versions collection.

Vivid status colors
-------------------
Card backgrounds were too close (dark gray vs slightly bluer dark gray).
New scheme:
- Installed: card stays dark gray, but a 4px green strip on the left and
  a vivid green "● Installée" pill make it unmistakable.
- Available remote-only: distinct dark blue card + vivid blue strip + blue
  "○ Disponible" pill.
- Busy: amber strip + amber pill, amber progress bar.

Brushes added to Theme.xaml: Brush.Status.Installed (#16A34A),
Brush.Status.Available (#3B82F6), Brush.Status.Busy (#F59E0B), each with
a matching very-dark-tint *Bg variant for card backgrounds.

Markdown theming
----------------
Release notes dialogs were unreadable (white background, near-white text).
Markdig.Wpf produces a FlowDocument with default white bg / black fg
regardless of the host control. Added MarkdownTheming.BuildThemedDocument
which renders the Markdown then walks all blocks/inlines to apply the
launcher's dark palette (Brush.Bg.Card transparent, Brush.Text.Primary
foreground, Brush.Accent links). Both UpdateAvailableDialog and
ReleaseNotesViewerDialog use this helper.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/PSLauncher.App/ViewModels/MainViewModel.cs

@@ -32,7 +32,16 @@ public sealed partial class MainViewModel : ObservableObject
     private CancellationTokenSource? _activeDownloadCts;
     private VersionRowViewModel? _activeRow;
 
-    public ObservableCollection<VersionRowViewModel> Versions { get; } = new();
+    public ObservableCollection<VersionRowViewModel> OtherVersions { get; } = new();
+
+    [ObservableProperty]
+    [NotifyPropertyChangedFor(nameof(HasFeaturedVersion))]
+    [NotifyPropertyChangedFor(nameof(HasOtherVersions))]
+    [NotifyPropertyChangedFor(nameof(EmptyHintVisibility))]
+    private VersionRowViewModel? _featuredVersion;
+
+    public bool HasFeaturedVersion => FeaturedVersion is not null;
+    public bool HasOtherVersions => OtherVersions.Count > 0;
 
     [ObservableProperty]
     [NotifyPropertyChangedFor(nameof(FooterText))]
@@ -57,7 +66,8 @@ public sealed partial class MainViewModel : ObservableObject
         _config.InstallRoot;
 
     public Visibility EmptyHintVisibility =>
-        Versions.Count == 0 ? Visibility.Visible : Visibility.Collapsed;
+        FeaturedVersion is null && OtherVersions.Count == 0
+            ? Visibility.Visible : Visibility.Collapsed;
 
     public Visibility FooterVisibility =>
         IsBusy || !string.IsNullOrEmpty(StatusMessage) ? Visibility.Visible : Visibility.Collapsed;
@@ -89,7 +99,11 @@ public sealed partial class MainViewModel : ObservableObject
         RebuildList();
     }
 
-    /// <summary>Recompose la liste affichée à partir du scan local + du dernier manifest distant.</summary>
+    /// <summary>
+    /// Recompose la liste affichée à partir du scan local + du dernier manifest distant.
+    /// La version « courante » (FeaturedVersion) est la plus haute installée ; à défaut,
+    /// la plus haute disponible côté serveur. Toutes les autres vont dans OtherVersions.
+    /// </summary>
     private void RebuildList()
     {
         var installed = _registry.Scan().ToDictionary(v => v.Version);
@@ -100,7 +114,7 @@ public sealed partial class MainViewModel : ObservableObject
             .OrderByDescending(v => SemVer.Parse(v))
             .ToList();
 
-        Versions.Clear();
+        var rows = new List<VersionRowViewModel>();
         foreach (var ver in allVersions)
         {
             VersionRowViewModel row;
@@ -113,11 +127,24 @@ public sealed partial class MainViewModel : ObservableObject
                 row = VersionRowViewModel.ForRemote(remoteByVer[ver]);
             }
             WireRowHandlers(row);
-            Versions.Add(row);
+            rows.Add(row);
+        }
+
+        // Featured : plus haute installée, sinon plus haute distante
+        var featured = rows.FirstOrDefault(r => r.IsInstalled) ?? rows.FirstOrDefault();
+        FeaturedVersion = featured;
+
+        OtherVersions.Clear();
+        foreach (var r in rows)
+        {
+            if (!ReferenceEquals(r, featured))
+                OtherVersions.Add(r);
         }
 
         OnPropertyChanged(nameof(EmptyHint));
         OnPropertyChanged(nameof(EmptyHintVisibility));
+        OnPropertyChanged(nameof(HasFeaturedVersion));
+        OnPropertyChanged(nameof(HasOtherVersions));
     }
 
     private void WireRowHandlers(VersionRowViewModel row)