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/routes/stammbaum/+page.svelte

@@ -5,6 +5,7 @@ import { page } from '$app/state';
 import { replaceState } from '$app/navigation';
 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 {
 	type PanZoomState,
@@ -127,16 +128,12 @@ $effect(() => {
 						onClose={() => (selectedId = null)}
 					/>
 				</aside>
-				<!-- Mobile: fixed bottom sheet -->
-				<div
-					class="fixed inset-x-0 bottom-0 z-40 max-h-[60dvh] overflow-y-auto border-t border-line bg-surface shadow-lg md:hidden"
-				>
-					<StammbaumSidePanel
-						node={selectedNode}
-						canWrite={canWrite}
-						onClose={() => (selectedId = null)}
-					/>
-				</div>
+				<!-- Mobile: dismissible bottom sheet (overlay, preserves pan/zoom) -->
+				<StammbaumBottomSheet
+					node={selectedNode}
+					canWrite={canWrite}
+					onClose={() => (selectedId = null)}
+				/>
 			{/if}
 		</div>
 	{/if}