feat(ui): spinners for ConPTY session start and task refine

Both actions previously gave no feedback: opening a ConPTY session only created
the tile after the launch-spec roundtrip (which may build a worktree), and the
refine button just disappeared while the run was in flight.

Add a shared Ellipse.spinner style, and let ConPtyPaneViewModel resolve its own
launch spec so the tile shows up immediately with a starting overlay. A failed
launch now keeps the tile with its inline error banner instead of never
appearing — Start() is separated from the ctor so the host can subscribe to
ErrorReported before the launch begins.

src/ClaudeDo.Ui/Views/Islands/TaskRowView.axaml

@@ -207,17 +207,23 @@
           </StackPanel>
         </StackPanel>
 
-        <!-- Refine button -->
-        <Button Grid.Column="5" Classes="icon-btn refine-btn"
-                IsVisible="{Binding CanRefine}"
-                VerticalAlignment="Top" Margin="0,2,0,0"
-                Command="{Binding $parent[ItemsControl].((vm:TasksIslandViewModel)DataContext).RefineTaskCommand}"
-                CommandParameter="{Binding}"
-                ToolTip.Tip="{loc:Tr tasks.refineTip}">
-          <Viewbox Width="16" Height="16">
-            <Path Classes="plan-icon" Data="{StaticResource Icon.Refine}"/>
-          </Viewbox>
-        </Button>
+        <!-- Refine button, replaced by a spinner while the refine run is in flight -->
+        <Panel Grid.Column="5">
+          <Button Classes="icon-btn refine-btn"
+                  IsVisible="{Binding CanRefine}"
+                  VerticalAlignment="Top" Margin="0,2,0,0"
+                  Command="{Binding $parent[ItemsControl].((vm:TasksIslandViewModel)DataContext).RefineTaskCommand}"
+                  CommandParameter="{Binding}"
+                  ToolTip.Tip="{loc:Tr tasks.refineTip}">
+            <Viewbox Width="16" Height="16">
+              <Path Classes="plan-icon" Data="{StaticResource Icon.Refine}"/>
+            </Viewbox>
+          </Button>
+          <Ellipse Classes="spinner"
+                   IsVisible="{Binding IsRefining}"
+                   VerticalAlignment="Top" Margin="0,4,2,0"
+                   ToolTip.Tip="{loc:Tr tasks.refiningTip}"/>
+        </Panel>
 
         <!-- Star toggle -->
         <Button Grid.Column="6" Classes="icon-btn star-btn"