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/routes/stammbaum/+page.svelte

@@ -7,6 +7,7 @@ import StammbaumTree from '$lib/person/genealogy/StammbaumTree.svelte';
 import StammbaumSidePanel from '$lib/person/genealogy/StammbaumSidePanel.svelte';
 import StammbaumBottomSheet from '$lib/person/genealogy/StammbaumBottomSheet.svelte';
 import StammbaumControls from '$lib/person/genealogy/StammbaumControls.svelte';
+import StammbaumAffordance from '$lib/person/genealogy/StammbaumAffordance.svelte';
 import {
 	type PanZoomState,
 	DEFAULT_VIEW,
@@ -36,6 +37,7 @@ let selectedId = $state<string | null>(
 const selectedNode = $derived(data.nodes.find((n) => n.id === selectedId) ?? null);
 
 let view = $state<PanZoomState>(data.initialView);
+let canvasActivity = $state(false);
 function zoomIn() {
 	view = { ...view, z: clampZoom(view.z + ZOOM_STEP_KB) };
 }
@@ -124,8 +126,10 @@ $effect(() => {
 					panZoom={view}
 					centreOnId={centreOnId}
 					onPanZoom={(v) => (view = v)}
+					onActivity={() => (canvasActivity = true)}
 					onSelect={(id) => (selectedId = id)}
 				/>
+				<StammbaumAffordance dismissed={canvasActivity} />
 				<StammbaumControls onZoomIn={zoomIn} onZoomOut={zoomOut} onFit={fitToScreen} />
 			</div>
 			{#if selectedNode}