feat(ui): add compact desktop cursor portrait

lib/cursor-portrait.ts

@@ -0,0 +1,16 @@
+export const TOTAL_DURATION = 4;
+export const ACTIVE_START = 0.25;
+export const ACTIVE_END = 3.75;
+export const DEFAULT_TIME = 2;
+export const SMOOTHING = 0.12;
+
+export function clamp(value: number, minimum: number, maximum: number) {
+  return Math.min(Math.max(value, minimum), maximum);
+}
+
+export function mapPointerYToTime(pointerY: number, viewportHeight: number) {
+  if (viewportHeight <= 0) return DEFAULT_TIME;
+
+  const progress = clamp(1 - pointerY / viewportHeight, 0, 1);
+  return ACTIVE_START + progress * (ACTIVE_END - ACTIVE_START);
+}