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/session/create-session-form.tsx

@@ -1,5 +1,6 @@
 import { useState, useEffect } from "react";
 import { Icon } from "../../icon";
+import { LoadingOverlay } from "../../loading-overlay";
 import {
 	createInstance,
 	startInstance,
@@ -215,6 +216,7 @@ export const CreateSessionForm = ({
 
 	return (
 		<div className={`create-session-form-wrapper ${className}`}>
+			<LoadingOverlay visible={isSubmitting} label="Creating session..." />
 			<form onSubmit={handleSubmit} className="stack create-session-form">
 				{/* Project */}
 				<div className="form-field">

apps/web/src/components/features/session/session-card.tsx

@@ -3,6 +3,7 @@ import type { Session } from "../../../api/sessions";
 import { Icon } from "../../icon";
 import { useMobileViewport } from "../../../hooks/use-mobile-viewport";
 import { MobileActionSheet } from "../mobile/mobile-action-sheet";
+import { LoadingOverlay } from "../../loading-overlay";
 import type { IconName } from "../../icon";
 
 export interface SessionCardProps {
@@ -14,6 +15,7 @@ export interface SessionCardProps {
 	onRecreateTunnel?: (session: Session) => void;
 	onRename?: (session: Session, newName: string) => void;
 	isBusy?: boolean;
+	busyLabel?: string;
 	tunnelHealth?: {
 		healthy: boolean;
 		container_status: string;
@@ -46,6 +48,7 @@ export function SessionCard({
 	onRecreateTunnel,
 	onRename,
 	isBusy = false,
+	busyLabel = "Working...",
 	tunnelHealth = null,
 }: SessionCardProps) {
 	const [showActionSheet, setShowActionSheet] = useState(false);
@@ -106,6 +109,7 @@ export function SessionCard({
 
 	return (
 		<article className="card session-card">
+			<LoadingOverlay visible={isBusy} label={busyLabel} />
 			<div className="session-card-content">
 				<div className="session-card-header">
 					<div className="session-card-title">