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/StammbaumTree.svelte.test.ts

@@ -2,6 +2,7 @@ import { describe, it, expect, vi } from 'vitest';
 import { render } from 'vitest-browser-svelte';
 import StammbaumTree from './StammbaumTree.svelte';
 import type { PanZoomState } from './panZoom';
+import { DIMMED_OPACITY } from './layout/highlightLineage';
 
 const ID_A = '00000000-0000-0000-0000-000000000001';
 const ID_B = '00000000-0000-0000-0000-000000000002';
@@ -962,7 +963,7 @@ describe('StammbaumTree lineage highlight (#703)', () => {
 		if (!fill) throw new Error(`No card-fill rect rendered for ${displayName}`);
 		return fill.getAttribute('opacity') === null && fill.getAttribute('fill-opacity') === null;
 	}
-	const DIM = '0.4';
+	const DIM = String(DIMMED_OPACITY);
 
 	it('renders every node at full strength when nothing is selected (AC1)', () => {
 		render(StammbaumTree, {