feat(stammbaum): dim a node when outside the highlighted lineage (#703)

StammbaumNode gains an optional `dimmed` prop that sets group-level
opacity (DIMMED_OPACITY) on the node's root <g>, so the box, accent bar,
name, and dates fade together as one unit. A lineage-fade CSS transition
eases the change and is neutralised under prefers-reduced-motion. The
selected-node styling (active fill + mint accent bar) is untouched.

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

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

@@ -1,5 +1,6 @@
 <script lang="ts">
 import { NODE_W, NODE_H } from '$lib/person/genealogy/layout/buildLayout';
+import { DIMMED_OPACITY } from '$lib/person/genealogy/layout/highlightLineage';
 import type { components } from '$lib/generated/api';
 
 type PersonNodeDTO = components['schemas']['PersonNodeDTO'];
@@ -8,10 +9,12 @@ interface Props {
 	node: PersonNodeDTO;
 	pos: { x: number; y: number };
 	selected: boolean;
+	/** Dim the whole node when a lineage is highlighted and this person is outside it. */
+	dimmed?: boolean;
 	onSelect: (id: string) => void;
 }
 
-let { node, pos, selected, onSelect }: Props = $props();
+let { node, pos, selected, dimmed = false, onSelect }: Props = $props();
 
 // Each node owns its own focus-ring state (the focus ring is decorative; the
 // `<g role="button">` is the real focus target).
@@ -35,11 +38,12 @@ const datesLabel = $derived(
 	aria-label="{node.displayName}{datesLabel}"
 	aria-expanded={selected}
 	transform="translate({pos.x}, {pos.y})"
+	opacity={dimmed ? DIMMED_OPACITY : undefined}
 	onclick={() => onSelect(node.id)}
 	onkeydown={handleKey}
 	onfocus={() => (focused = true)}
 	onblur={() => (focused = false)}
-	class="cursor-pointer focus:outline-none"
+	class="lineage-fade cursor-pointer focus:outline-none"
 >
 	{#if focused}
 		<rect
@@ -88,3 +92,15 @@ const datesLabel = $derived(
 		</text>
 	{/if}
 </g>
+
+<style>
+/* Ease the lineage focus+dim transition; instant for reduced-motion users. */
+.lineage-fade {
+	transition: opacity 200ms ease;
+}
+@media (prefers-reduced-motion: reduce) {
+	.lineage-fade {
+		transition: none;
+	}
+}
+</style>