chore(lyrics): rename springScroll.ts to easeScroll.ts

After PR #214 replaced SpringScroller with EaseScroller, the filename
no longer matched the exported class. Rename for clarity.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/utils/easeScroll.ts

@@ -0,0 +1,69 @@
+/**
+ * Duration-based ease-out scroll animator.
+ *
+ * Animates scrollTop from the current position to the target over a fixed
+ * duration using a cubic ease-out curve. Calling scrollTo() mid-flight
+ * restarts cleanly from wherever the container currently sits, so fast
+ * line changes never look jerky or skip.
+ */
+export class EaseScroller {
+  private container : HTMLElement;
+  private startY    = 0;
+  private targetY   = 0;
+  private startTime = 0;
+  private rafId: number | null = null;
+
+  private readonly duration: number;
+
+  constructor(container: HTMLElement, duration = 650) {
+    this.container = container;
+    this.targetY   = container.scrollTop;
+    this.duration  = duration;
+  }
+
+  scrollTo(y: number) {
+    this.startY    = this.container.scrollTop;
+    this.targetY   = Math.max(0, y);
+    this.startTime = performance.now();
+    if (this.rafId === null) this.rafId = requestAnimationFrame(this.tick);
+  }
+
+  stop() {
+    if (this.rafId !== null) {
+      cancelAnimationFrame(this.rafId);
+      this.rafId = null;
+    }
+  }
+
+  jump(y: number) {
+    this.stop();
+    this.container.scrollTop = y;
+    this.targetY = y;
+  }
+
+  private tick = (now: number) => {
+    const t    = Math.min((now - this.startTime) / this.duration, 1);
+    const ease = 1 - Math.pow(1 - t, 3); // cubic ease-out
+    this.container.scrollTop = this.startY + (this.targetY - this.startY) * ease;
+    if (t < 1) {
+      this.rafId = requestAnimationFrame(this.tick);
+    } else {
+      this.container.scrollTop = this.targetY;
+      this.rafId = null;
+    }
+  };
+}
+
+/**
+ * Compute the scroll position that places `el` at `fraction` from the top
+ * of `container` (0 = top edge, 0.35 = Apple Music-style, 0.5 = centre).
+ */
+export function targetForFraction(
+  container: HTMLElement,
+  el       : HTMLElement,
+  fraction  = 0.35,
+): number {
+  const cRect = container.getBoundingClientRect();
+  const eRect = el.getBoundingClientRect();
+  return container.scrollTop + (eRect.top - cRect.top) - cRect.height * fraction + eRect.height / 2;
+}