feat(fitness): support repeat groups in interval training templates

Allow users to nest steps inside a repeat group (e.g. "5×: 30s sprint,
60s recovery") when building GPS interval templates. Groups are tagged
{ type: 'group', repeat, steps } alongside flat { type: 'step' } entries
and capped at one nesting level. Entries are expanded into a flat list
before handing off to the native Android TTS/interval service, so the
runtime state machine is unchanged.

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "homepage",
-	"version": "1.30.0",
+	"version": "1.31.0",
 	"private": true,
 	"type": "module",
 	"scripts": {