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/features/tool/instance-list.tsx

@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useState } from "react";
 import { useNavigate } from "react-router-dom";
 import { Icon } from "../../icon";
+import { LoadingOverlay } from "../../loading-overlay";
 import type { ToolInstance } from "../../../api/sessions";
 import {
 	deleteInstance,
@@ -60,6 +61,7 @@ export const InstanceList = ({
 
 	// Per-instance busy state for actions
 	const [busyInstanceId, setBusyInstanceId] = useState<string | null>(null);
+	const [busyLabel, setBusyLabel] = useState("Working...");
 
 	const loadInstances = useCallback(async () => {
 		setLoading(true);
@@ -135,6 +137,7 @@ export const InstanceList = ({
 		sshKeyIds?: string[],
 	) => {
 		setBusyInstanceId(instanceId);
+		setBusyLabel("Starting...");
 		try {
 			await startInstance(
 				projectId,
@@ -156,6 +159,7 @@ export const InstanceList = ({
 
 	const handleStop = async (instanceId: string) => {
 		setBusyInstanceId(instanceId);
+		setBusyLabel("Stopping...");
 		try {
 			await stopInstance(projectId, repoId, instanceId);
 			setStopConfirmId(null);
@@ -173,6 +177,7 @@ export const InstanceList = ({
 		sshKeyIds?: string[],
 	) => {
 		setBusyInstanceId(instanceId);
+		setBusyLabel("Restarting...");
 		try {
 			await restartInstance(
 				projectId,
@@ -195,6 +200,7 @@ export const InstanceList = ({
 	const handleDelete = async (instanceId: string) => {
 		if (!confirm("Are you sure you want to delete this instance?")) return;
 		setBusyInstanceId(instanceId);
+		setBusyLabel("Deleting...");
 		try {
 			await deleteInstance(projectId, repoId, instanceId);
 			// Update state immediately instead of reloading
@@ -250,6 +256,10 @@ export const InstanceList = ({
 				<div className="instance-grid">
 					{instances.map((instance) => (
 						<div key={instance.id} className="instance-card">
+							<LoadingOverlay
+								visible={busyInstanceId === instance.id}
+								label={busyLabel}
+							/>
 							<div className="instance-info">
 								<div className="instance-name">{instance.display_name}</div>
 								<div className="instance-meta">