feat(stammbaum): dismissible accessible mobile bottom sheet (#692)

Wrap the mobile person panel in StammbaumBottomSheet: drag-handle grip with
swipe-down-to-dismiss (≥80px), full-screen backdrop button for tap-outside
dismiss, role=dialog + aria-label, focus trap, and Escape (NFR-A11Y-004).
Pan/zoom state is untouched by open/close (US-PANEL-001/002).

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

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

@@ -0,0 +1,74 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+import { trapFocus } from '$lib/shared/actions/trapFocus';
+import StammbaumSidePanel from '$lib/person/genealogy/StammbaumSidePanel.svelte';
+import type { components } from '$lib/generated/api';
+
+type PersonNodeDTO = components['schemas']['PersonNodeDTO'];
+
+interface Props {
+	node: PersonNodeDTO;
+	canWrite: boolean;
+	onClose: () => void;
+}
+
+let { node, canWrite, onClose }: Props = $props();
+
+// Swipe the sheet down past this threshold to dismiss it (Leonie).
+const SWIPE_DISMISS_PX = 80;
+let dragY = $state(0);
+let dragging = false;
+let startY = 0;
+
+function onHandleDown(event: PointerEvent) {
+	dragging = true;
+	startY = event.clientY;
+	(event.currentTarget as HTMLElement).setPointerCapture?.(event.pointerId);
+}
+function onHandleMove(event: PointerEvent) {
+	if (dragging) dragY = Math.max(0, event.clientY - startY);
+}
+function onHandleUp() {
+	if (!dragging) return;
+	dragging = false;
+	if (dragY >= SWIPE_DISMISS_PX) onClose();
+	dragY = 0;
+}
+
+function onKeydown(event: KeyboardEvent) {
+	if (event.key === 'Escape') onClose();
+}
+</script>
+
+<!-- Backdrop: a full-screen button so tap-outside dismiss is keyboard- and
+     screen-reader-accessible without a static-element click handler. -->
+<button
+	type="button"
+	class="fixed inset-0 z-30 bg-black/30 md:hidden"
+	aria-label={m.stammbaum_close_panel()}
+	onclick={onClose}
+></button>
+
+<div
+	role="dialog"
+	aria-modal="true"
+	aria-label={node.displayName}
+	class="fixed inset-x-0 bottom-0 z-40 max-h-[60dvh] overflow-y-auto rounded-t-xl border-t border-line bg-surface shadow-lg md:hidden"
+	style="transform: translateY({dragY}px);"
+	use:trapFocus
+	onkeydown={onKeydown}
+>
+	<!-- Drag handle grip — swipe down to dismiss. -->
+	<!-- svelte-ignore a11y_no_static_element_interactions -->
+	<div
+		class="flex cursor-grab justify-center py-2 active:cursor-grabbing"
+		onpointerdown={onHandleDown}
+		onpointermove={onHandleMove}
+		onpointerup={onHandleUp}
+		onpointercancel={onHandleUp}
+	>
+		<div class="h-1 w-10 rounded-full bg-line" aria-hidden="true"></div>
+	</div>
+
+	<StammbaumSidePanel node={node} canWrite={canWrite} onClose={onClose} />
+</div>