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>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "ruwio",
   "private": true,
-  "version": "0.13.0",
+  "version": "0.13.1",
   "type": "module",
   "scripts": {
     "dev": "vite",