feat(stammbaum): first-load touch affordance hint (#692)

Add StammbaumAffordance: a touch-only "drag to explore · pinch to zoom" hint
that auto-dismisses on the first canvas pointer interaction (wired via the
gesture action's onGestureStart) or the explicit close, and stays dismissed for
30 days via a localStorage timestamp (boolean gate only, never rendered).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/StammbaumTree.svelte

@@ -29,6 +29,8 @@ interface Props {
 	onPanZoom?: (state: PanZoomState) => void;
 	/** When set to a node id, the canvas recentres on that node (US-PAN-005). */
 	centreOnId?: string | null;
+	/** Fired on the first pointer interaction with the canvas (affordance dismiss). */
+	onActivity?: () => void;
 	onSelect: (id: string) => void;
 	/**
 	 * Force-show or force-hide the generation gutter. When undefined, falls
@@ -46,6 +48,7 @@ let {
 	panZoom,
 	onPanZoom = () => {},
 	centreOnId = null,
+	onActivity,
 	onSelect,
 	showGutter
 }: Props = $props();
@@ -295,7 +298,8 @@ const parentLinks = $derived.by<ParentLinks>(() => {
 		baseCentreX: baseCentre.x,
 		baseCentreY: baseCentre.y,
 		reducedMotion,
-		onPanZoom
+		onPanZoom,
+		onGestureStart: onActivity
 	}}
 	class="block h-full w-full"
 >