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 });
+	});
+});

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

@@ -61,3 +61,23 @@ export function parsePanZoomParams(raw: {
 export function serializePanZoomParams(state: PanZoomState): { cx: string; cy: string; z: string } {
 	return { cx: String(state.x), cy: String(state.y), z: String(state.z) };
 }
+
+/**
+ * Convert a pointer delta in CSS pixels into SVG user units, using the current
+ * viewBox-to-element ratio per axis. This is the distance the pointer traversed
+ * expressed in the tree's coordinate space; the gesture handler subtracts it
+ * from the pan offset so the canvas follows the finger (US-PAN-001).
+ */
+export function screenDeltaToSvg(
+	dxPx: number,
+	dyPx: number,
+	viewBoxW: number,
+	viewBoxH: number,
+	elPxW: number,
+	elPxH: number
+): { dx: number; dy: number } {
+	return {
+		dx: elPxW > 0 ? dxPx * (viewBoxW / elPxW) : 0,
+		dy: elPxH > 0 ? dyPx * (viewBoxH / elPxH) : 0
+	};
+}