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.

src/lib/js/gps.svelte.ts

@@ -15,11 +15,39 @@ export interface GpsPoint {
 }
 
 export interface IntervalStep {
+	type?: 'step';
 	label: string;
 	durationType: 'distance' | 'time';
 	durationValue: number; // meters (distance) or seconds (time)
 }
 
+export interface IntervalGroup {
+	type: 'group';
+	repeat: number;
+	steps: IntervalStep[];
+}
+
+export type IntervalEntry = IntervalStep | IntervalGroup;
+
+/** Expand groups (repeat × steps) into a flat step list for the native bridge. */
+export function flattenIntervals(entries: IntervalEntry[]): IntervalStep[] {
+	const out: IntervalStep[] = [];
+	for (const e of entries) {
+		if ((e as IntervalGroup).type === 'group') {
+			const g = e as IntervalGroup;
+			for (let i = 0; i < g.repeat; i++) {
+				for (const s of g.steps) {
+					out.push({ label: s.label, durationType: s.durationType, durationValue: s.durationValue });
+				}
+			}
+		} else {
+			const s = e as IntervalStep;
+			out.push({ label: s.label, durationType: s.durationType, durationValue: s.durationValue });
+		}
+	}
+	return out;
+}
+
 export interface VoiceGuidanceConfig {
 	enabled: boolean;
 	triggerType: 'distance' | 'time';