0.13.1: new track-group lands above the active layer

addTrackGroup takes an optional anchorTrackId and inserts directly
above it in trackOrder; anchors nested inside another group resolve
to their parent's slot. The timeline's group-add button forwards
selectedLayerId so the new group lands where the user is working
instead of always jumping to the top. Covered by store + UI tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/Timeline.vue

@@ -1592,7 +1592,10 @@ function toggleGroupCollapsed(groupId: string) {
 }
 
 function addGroup() {
-  store.addTrackGroup()
+  // Drop the group directly above whatever the user is currently
+  // working on — layer selection is the only active-track signal
+  // today. Falls back to the top of the stack when nothing's active.
+  store.addTrackGroup(undefined, store.selectedLayerId ?? null)
 }
 
 async function renameGroupPrompt(groupId: string, current: string) {