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/Design/IslandStyles.axaml

@@ -454,6 +454,25 @@
     </Style.Animations>
   </Style>
 
+  <!-- Indeterminate ring spinner (dashed ring, rotated forever) -->
+  <Style Selector="Ellipse.spinner">
+    <Setter Property="Width"           Value="14" />
+    <Setter Property="Height"          Value="14" />
+    <Setter Property="Stroke"          Value="{StaticResource AccentBrush}" />
+    <Setter Property="StrokeThickness" Value="2" />
+    <Setter Property="StrokeDashArray" Value="3,2" />
+    <Setter Property="RenderTransform" Value="rotate(0deg)" />
+    <Style.Animations>
+      <Animation Duration="0:0:0.9" IterationCount="INFINITE" Easing="LinearEasing">
+        <KeyFrame Cue="0%">  <Setter Property="RotateTransform.Angle" Value="0" />  </KeyFrame>
+        <KeyFrame Cue="100%"><Setter Property="RotateTransform.Angle" Value="360" /></KeyFrame>
+      </Animation>
+    </Style.Animations>
+  </Style>
+  <Style Selector="Ellipse.spinner.dim">
+    <Setter Property="Stroke" Value="{StaticResource TextDimBrush}" />
+  </Style>
+
   <!-- ============================================================ -->
   <!-- AGENT STRIP                                                  -->
   <!-- ============================================================ -->