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/MissionControl/ConPtyPaneView.axaml

@@ -44,8 +44,20 @@
                    TextWrapping="Wrap" />
       </Border>
 
-      <!-- Embedded ConPTY terminal -->
-      <views:InteractiveTerminalView DataContext="{Binding Terminal}" />
+      <!-- Embedded ConPTY terminal, with a starting overlay until the session is spawned -->
+      <Panel>
+        <views:InteractiveTerminalView DataContext="{Binding Terminal}" />
+        <Border IsVisible="{Binding Terminal.IsStarting}"
+                Background="{DynamicResource VoidBrush}">
+          <StackPanel Orientation="Horizontal" Spacing="10"
+                      HorizontalAlignment="Center" VerticalAlignment="Center">
+            <Ellipse Classes="spinner"/>
+            <TextBlock Classes="meta" Text="{loc:Tr missionControl.conptyStarting}"
+                       Foreground="{DynamicResource TextDimBrush}"
+                       VerticalAlignment="Center"/>
+          </StackPanel>
+        </Border>
+      </Panel>
 
     </DockPanel>
   </Border>