feat(stammbaum): /stammbaum page — SVG tree + side panel + empty state

- /stammbaum/+page.server.ts loads GET /api/network (already filtered
  to family members on the backend) and returns nodes + edges.
- +page.svelte holds the page shell, manages selectedId (with
  ?focus={id} deep-link support) and zoom state, renders the empty
  state when nodes.length === 0 (icon + heading + body + link to
  /persons), or the tree + side panel otherwise.
- StammbaumTree.svelte: BFS-based generation assignment from roots,
  spouses promoted to the deeper generation so couples sit on the same
  row, alphabetical sort within row, simple grid layout. SVG nodes are
  role="button" + aria-label="{name}, {birth}–{death}" +
  aria-expanded={selected}, with click + Enter/Space activation. Solid
  parent→child connectors; mint spouse line with midpoint circle, dashed
  if SPOUSE_OF.toYear is set (former spouse). Zoom maps to viewBox.
- StammbaumSidePanel.svelte: lazily loads
  /api/persons/{id}/relationships and /inferred-relationships when the
  selection changes; shows direct chips (mint), top-5 derived chips
  (grey), and a "Zur Personenseite →" link. Escape closes the panel.

Refs #358.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/stammbaum/+page.server.ts

@@ -0,0 +1,18 @@
+import { error, redirect } from '@sveltejs/kit';
+import { createApiClient } from '$lib/api.server';
+import { getErrorMessage } from '$lib/errors';
+
+export async function load({ fetch }) {
+	const api = createApiClient(fetch);
+	const result = await api.GET('/api/network');
+
+	if (result.response.status === 401) throw redirect(302, '/login');
+
+	if (!result.response.ok) {
+		const code = (result.error as unknown as { code?: string })?.code;
+		throw error(result.response.status, getErrorMessage(code));
+	}
+
+	const network = result.data!;
+	return { nodes: network.nodes ?? [], edges: network.edges ?? [] };
+}