feat(review): gate Approve & Merge behind opening the diff

Approve & Merge is disabled until the pending changes have been inspected:
DetailsIslandViewModel tracks ReviewDiffViewed (reset on task switch and on
every state change), MergeSectionViewModel raises DiffViewed when a diff or
combined diff is opened and exposes HasReviewableDiff, and a hint sits next
to the button. A review with nothing to inspect (sandbox run, no worktree)
approves straight through.

ClaudeDo-Task: f9809a93

src/ClaudeDo.Ui/Design/IslandStyles.axaml

@@ -1162,6 +1162,13 @@
     <Setter Property="BorderBrush" Value="{StaticResource AccentBrush}" />
     <Setter Property="TextElement.Foreground" Value="{StaticResource TextBrush}" />
   </Style>
+  <!-- Disabled: drop the accent so a gated action (e.g. Approve before the diff is
+       opened) reads clearly as unavailable instead of looking clickable. -->
+  <Style Selector="Button.accent:disabled /template/ ContentPresenter">
+    <Setter Property="Background"  Value="{StaticResource Surface2Brush}" />
+    <Setter Property="BorderBrush" Value="{StaticResource LineBrush}" />
+    <Setter Property="TextElement.Foreground" Value="{StaticResource TextMuteBrush}" />
+  </Style>
 
   <!-- ============================================================ -->
   <!-- DAY TOGGLE                                                    -->