fix(ui): diff viewer opens non-modal and maximizable

The diff viewer used ShowDialog at both call sites (task details, worktrees
overview), blocking navigation of the rest of the app while open. Switch both
to Window.Show(owner) and reuse the one open window (swap DataContext,
Activate()) instead of stacking a second one on re-open.

Add an opt-in maximize/restore button + double-click-titlebar toggle to
ModalShell (ShowMaximizeButton, default false so other modals are untouched),
wired on for DiffViewerView only. Reuses the existing Icon.WinMax/WinRestore
geometry and OffScreenMargin inset from MainWindow's own maximize handling.

The review gate (Approve & Merge locked until the diff is opened) already
fires DiffViewed before the dialog/window is shown, so it is unaffected by
the modal-to-non-modal change.

src/ClaudeDo.Ui/Views/Controls/ModalShell.axaml

@@ -14,14 +14,19 @@
                     Background="{DynamicResource DeepBrush}"
                     BorderBrush="{DynamicResource LineBrush}"
                     BorderThickness="0,0,0,1">
-              <Grid ColumnDefinitions="*,Auto" Margin="14,0">
+              <Grid ColumnDefinitions="*,Auto,Auto" Margin="14,0">
                 <TextBlock Text="{TemplateBinding Title}"
                            FontFamily="{DynamicResource MonoFont}"
                            FontSize="{DynamicResource FontSizeMono}"
                            LetterSpacing="1.4"
                            Foreground="{DynamicResource TextBrush}"
                            VerticalAlignment="Center"/>
-                <Button Grid.Column="1" Classes="icon-btn" Content="✕"
+                <Button Name="PART_MaxButton" Grid.Column="1" Classes="icon-btn"
+                        IsVisible="{TemplateBinding ShowMaximizeButton}"
+                        VerticalAlignment="Center">
+                  <PathIcon Name="PART_MaxIcon" Data="{StaticResource Icon.WinMax}" Width="10" Height="10"/>
+                </Button>
+                <Button Grid.Column="2" Classes="icon-btn" Content="✕"
                         FontSize="{DynamicResource FontSizeBody}"
                         Command="{TemplateBinding CloseCommand}"
                         VerticalAlignment="Center"/>