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/styles/utilities.css

@@ -383,6 +383,51 @@ a.nav-item,
 	align-items: center;
 }
 
+/* ============================================
+   Loading Overlay
+   ============================================ */
+
+.loading-overlay {
+	position: absolute;
+	inset: 0;
+	background: rgba(0, 0, 0, 0.12);
+	backdrop-filter: blur(1px);
+	border-radius: inherit;
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	z-index: 10;
+}
+
+.loading-overlay-content {
+	display: flex;
+	flex-direction: column;
+	align-items: center;
+	gap: var(--space-2);
+	color: var(--text, currentColor);
+}
+
+.loading-overlay-label {
+	font-size: var(--text-sm);
+	font-weight: 500;
+}
+
+.icon-spin {
+	animation: icon-spin 1s linear infinite;
+}
+
+@keyframes icon-spin {
+	to {
+		transform: rotate(360deg);
+	}
+}
+
+.session-card,
+.tool-starter,
+.create-session-form-wrapper {
+	position: relative;
+}
+
 /* ============================================
    Terminal Styles
    ============================================ */