Panel busy states: Connecting placeholder with Cancel, slow-load overlay

Connecting blanks the list to a fading spinner with the target name and
a Cancel button (token discards late results and orphan sessions).
Listings >300ms dim the list under a spinner. All panel input is
blocked while busy; the source picker stays usable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/styles.css

@@ -188,6 +188,16 @@
   --action-icon: 1.05rem;
 }
 
+/* Soft entrance for the panel busy states (connecting placeholder, load overlay). */
+@keyframes panel-fade-in {
+  from {
+    opacity: 0;
+  }
+}
+.panel-fade-in {
+  animation: panel-fade-in 0.18s ease-out;
+}
+
 /* Accent tone (data-accent set by SettingsService; absent = graphite/zinc).
    Each tone is one hue+chroma line; the formula blocks below derive --primary,
    --ring and the cursor-row tint for light and both dark states. The dark