feat(stammbaum): convert pointer pixel delta to SVG units (#692)

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

frontend/src/lib/person/genealogy/panZoom.test.ts

@@ -3,6 +3,7 @@ import {
 	clampZoom,
 	parsePanZoomParams,
 	serializePanZoomParams,
+	screenDeltaToSvg,
 	DEFAULT_VIEW,
 	DEFAULT_ZOOM,
 	MIN_ZOOM,
@@ -75,3 +76,18 @@ describe('serializePanZoomParams', () => {
 		expect(parsePanZoomParams(serializePanZoomParams(state))).toEqual(state);
 	});
 });
+
+describe('screenDeltaToSvg', () => {
+	it('scales a pixel delta by the viewBox-to-element ratio per axis', () => {
+		// viewBox is 2x the element in width, 2x in height → 1px == 2 SVG units.
+		expect(screenDeltaToSvg(10, 5, 1000, 800, 500, 400)).toEqual({ dx: 20, dy: 10 });
+	});
+
+	it('is identity when the viewBox matches the element pixel size', () => {
+		expect(screenDeltaToSvg(7, -3, 600, 600, 600, 600)).toEqual({ dx: 7, dy: -3 });
+	});
+
+	it('returns zero when the element has no measured size', () => {
+		expect(screenDeltaToSvg(10, 10, 1000, 800, 0, 0)).toEqual({ dx: 0, dy: 0 });
+	});
+});