feat: busy container overlays for lifecycle actions

Add a reusable LoadingOverlay component that dims and disables the
container owning an in-flight action, with a spinning indicator and
label. Apply it to:

- SessionCard (when actionBusyId matches)
- InstanceList cards (per busyInstanceId with action-specific labels)
- CreateSessionForm (while submitting)
- ToolStarter (while starting)

Also add .icon-spin animation and position:relative to the relevant
containers.

Quality gates: npm run typecheck, npm run lint, npm test -- --run
(87 passed).

apps/web/src/components/loading-overlay.tsx

@@ -0,0 +1,21 @@
+import { Icon } from "./icon";
+
+interface LoadingOverlayProps {
+	visible: boolean;
+	label?: string;
+}
+
+export function LoadingOverlay({ visible, label }: LoadingOverlayProps) {
+	if (!visible) return null;
+
+	return (
+		<div className="loading-overlay" aria-live="polite">
+			<div className="loading-overlay-content">
+				<Icon name="loading" size="lg" className="icon-spin" ariaLabel={label} />
+				{label && (
+					<span className="loading-overlay-label">{label}</span>
+				)}
+			</div>
+		</div>
+	);
+}