fix(ui): only show overlay when starting, not on error

The overlay covered the whole window in error phase too, blocking
access to the sidebar and Settings page where the user needs to
read the sidecar logs to diagnose the failure.

Now in error phase the sidebar status dot turns red, the start
button is back in the sidebar, and the Settings page is reachable
to inspect the in-app log buffer.

src/App.vue

@@ -126,8 +126,8 @@ async function handleStop() {
     </main>
 
     <StartupOverlay
-      v-if="phase !== 'ready' && phase !== 'idle'"
+      v-if="phase === 'starting'"
-      :phase="phase as 'starting' | 'error'"
+      phase="starting"
       :error="error"
       @start="handleStart"
       @retry="handleStart"