feat(stammbaum): inline add-relationship form in side panel

Implements the inline-edit affordance from
docs/specs/stammbaum-tree-spec.html (section 3): a low-opacity
"+ Beziehung hinzufügen" button below the direct relationships list
expands into a compact form (type select, person typeahead,
optional Von/Bis Jahr inputs, Abbrechen + Speichern). On save the
form POSTs to /api/persons/{id}/relationships, reloads the panel's
own data, and calls invalidateAll() so the tree picks up the new
edge without a hard refresh.

The panel takes a new canWrite prop, plumbed through from the
+layout.server.ts data already exposed on page.data.

Also pins the /stammbaum canvas to the viewport (-my-6 cancels
<main>'s py-6, h-[calc(100dvh-4.25rem)] subtracts the navbar) so
the page no longer overflows below the fold.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/routes/stammbaum/+page.svelte

@@ -15,6 +15,7 @@ interface Props {
 let { data }: Props = $props();
 
 const focusId = $derived(page.url.searchParams.get('focus'));
+const canWrite = $derived<boolean>(page.data.canWrite ?? false);
 
 let selectedId = $state<string | null>(null);
 $effect(() => {
@@ -34,7 +35,10 @@ function zoomOut() {
 }
 </script>
 
-<div class="-mt-6 flex h-full flex-col">
+<!-- 4.25rem = 4rem navbar (h-16) + 0.25rem accent strip (h-1).
+     -my-6 cancels <main>'s py-6 so the canvas sits flush against the navbar
+     on top and the viewport edge on the bottom. -->
+<div class="-my-6 flex h-[calc(100dvh-4.25rem)] flex-col">
 	<header
 		class="flex shrink-0 items-center justify-between border-b border-line bg-surface px-6 py-4"
 	>
@@ -102,7 +106,11 @@ function zoomOut() {
 			</div>
 			{#if selectedNode}
 				<aside class="w-[268px] shrink-0 overflow-y-auto border-l border-line bg-surface">
-					<StammbaumSidePanel node={selectedNode} onClose={() => (selectedId = null)} />
+					<StammbaumSidePanel
+						node={selectedNode}
+						canWrite={canWrite}
+						onClose={() => (selectedId = null)}
+					/>
 				</aside>
 			{/if}
 		</div>