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.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
+	};
+}