UI rework: per-row actions, drop sidebar

Replace the sidebar + hero + single big-button layout with a flat list of
per-version cards. Each card carries its own action button:
- Installed: ▶ Lancer (green, primary)
- Available on server only: ⬇ Installer (blue, accent) + lighter blue card
- Busy: inline mini progress bar with %, card tinted green

Each card also exposes a "..." menu (left-click opens it) with:
- Voir les release notes (works for installed and remote-only versions)
- Ouvrir le dossier (installed only)
- Supprimer cette version (installed only, with confirmation dialog)

VersionRowViewModel owns its state (InstalledIdle / AvailableIdle /
Downloading / Installing / Uninstalling) and its commands; MainViewModel
wires per-row handlers after instantiation so the row VM stays UI-only
and the services live one layer up.

ReleaseNotesViewerDialog: separate dialog reused by the menu — same
Markdown rendering as UpdateAvailableDialog but no download CTA.

Theme: AccentButton + IconButton + dark ContextMenu/MenuItem styles.

Behavior changes:
- The single global SelectedVersion + AvailableUpdate are gone; each row
  is independently actionable.
- The list now merges installed + remote: a version present on the
  server but not locally appears as a "remote-only" row, and vice-versa.

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

src/PSLauncher.App/Views/ReleaseNotesViewerDialog.xaml

@@ -0,0 +1,42 @@
+<Window x:Class="PSLauncher.App.Views.ReleaseNotesViewerDialog"
+        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        Title="Notes de version"
+        Width="640" Height="540"
+        MinWidth="480" MinHeight="400"
+        WindowStartupLocation="CenterOwner"
+        Background="{StaticResource Brush.Bg.Window}">
+    <Grid Margin="24">
+        <Grid.RowDefinitions>
+            <RowDefinition Height="Auto" />
+            <RowDefinition Height="*" />
+            <RowDefinition Height="Auto" />
+        </Grid.RowDefinitions>
+
+        <TextBlock Grid.Row="0"
+                   Text="{Binding Title}"
+                   FontSize="22" FontWeight="SemiBold"
+                   Margin="0,0,0,18"
+                   Foreground="{StaticResource Brush.Text.Primary}" />
+
+        <Border Grid.Row="1"
+                Background="{StaticResource Brush.Bg.Card}"
+                BorderBrush="{StaticResource Brush.Border}" BorderThickness="1"
+                CornerRadius="6"
+                Padding="16">
+            <FlowDocumentScrollViewer
+                Document="{Binding ReleaseNotesDocument}"
+                Foreground="{StaticResource Brush.Text.Primary}"
+                Background="Transparent" />
+        </Border>
+
+        <Button Grid.Row="2"
+                Style="{StaticResource SecondaryButton}"
+                Content="Fermer"
+                IsCancel="True"
+                IsDefault="True"
+                HorizontalAlignment="Right"
+                Margin="0,18,0,0"
+                Click="OnClose" />
+    </Grid>
+</Window>