fix(ui): Branch-Zeile im Detail-Header — Icon stroked rendern, ohne Task ausblenden

Icon.GitBranch ist Line-Art und wurde per PathIcon gefuellt (nur Knoten sichtbar).
Zusaetzlich brach {Binding Task.HasBranch} bei leerer Auswahl (Task=null) und
IsVisible fiel auf den Default true zurueck — FallbackValue=False blendet die
Zeile jetzt aus, solange kein Task gebunden ist.

src/ClaudeDo.Ui/Views/Islands/Detail/TaskHeaderBar.axaml

@@ -27,12 +27,17 @@
                Padding="0"/>
 
       <!-- Branch meta line: moved here from the task row's chip (A5) — the row no longer
-           shows the branch anywhere else. -->
+           shows the branch anywhere else. FallbackValue: with no task bound the path can't
+           resolve and IsVisible would fall back to its default (true). -->
       <StackPanel Orientation="Horizontal" Spacing="4" Margin="0,4,0,0"
-                  IsVisible="{Binding Task.HasBranch}">
-        <PathIcon Width="10" Height="10"
-                  Data="{StaticResource Icon.GitBranch}"
-                  Foreground="{DynamicResource TextDimBrush}"/>
+                  IsVisible="{Binding Task.HasBranch, FallbackValue=False}">
+        <!-- Icon.GitBranch is line-art: stroke-rendered, PathIcon would fill the lines away -->
+        <Viewbox Width="10" Height="10">
+          <Path Data="{StaticResource Icon.GitBranch}"
+                Stroke="{DynamicResource TextDimBrush}"
+                StrokeThickness="2.4"
+                StrokeLineCap="Round" StrokeJoin="Round"/>
+        </Viewbox>
         <TextBlock Classes="meta" Text="{Binding Task.Branch}"/>
       </StackPanel>
     </StackPanel>