fix(stammbaum): raise dimmed opacity to 0.45 and bind tests to the constant (#703)

Bump DIMMED_OPACITY 0.4 -> 0.45 so dimmed outlines/labels stay legible
against bg-surface in both themes (dark mode dims already-light mint, the
riskier case). Import the constant into StammbaumTree.svelte.test.ts so the
node-opacity assertions track it instead of a hard-coded '0.4'.

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

frontend/src/lib/person/genealogy/layout/highlightLineage.ts

@@ -16,11 +16,13 @@ import type { components } from '$lib/generated/api';
 type RelationshipDTO = components['schemas']['RelationshipDTO'];
 
 /**
- * Opacity applied to dimmed nodes and connectors. ~0.4 keeps names legible while
- * clearly de-emphasised, and works as a lightness cue in both themes (the colour
- * tokens are theme-aware) so the cue does not rely on hue (WCAG 1.4.1 / NFR-A11Y-001).
+ * Opacity applied to dimmed node outlines/labels and connectors. 0.45 keeps names
+ * legible against bg-surface in both themes (dark mode dims already-light mint, the
+ * riskier case) while clearly de-emphasised, and works as a lightness cue so the cue
+ * does not rely on hue (WCAG 1.4.1 / NFR-A11Y-001). The dim is applied to the node's
+ * outline + labels only — the card fill stays opaque, see StammbaumNode.svelte.
  */
-export const DIMMED_OPACITY = 0.4;
+export const DIMMED_OPACITY = 0.45;
 
 /** Adjacency index over the family graph, built once per edge set. */
 export type LineageIndex = {