fix(pdf): prevent continuous scroll kickback (#3990)

Update foliate-js to preserve the current intra-page scroll anchor when fixed-layout pages relayout or replace placeholders in scrolled mode, instead of snapping back with scrollIntoView().

Add regression coverage for the scroll-anchor restore path.

Closes #3876.

apps/readest-app/src/__tests__/document/fixed-layout-scroll-mode.test.ts

@@ -0,0 +1,55 @@
+import { describe, expect, it } from 'vitest';
+
+import { captureScrollModeAnchor, restoreScrollModeAnchor } from 'foliate-js/fixed-layout.js';
+
+describe('fixed-layout scroll mode anchor preservation', () => {
+  it('captures the current intra-page offset', () => {
+    const anchor = captureScrollModeAnchor(
+      [
+        { index: 0, top: 0, height: 1000 },
+        { index: 1, top: 1008, height: 1000 },
+      ],
+      1350,
+      1,
+    );
+
+    expect(anchor).toEqual({
+      index: 1,
+      fraction: 0.342,
+      scrollTop: 1350,
+    });
+  });
+
+  it('restores the same intra-page position after page sizes change', () => {
+    const anchor = captureScrollModeAnchor(
+      [
+        { index: 0, top: 0, height: 1000 },
+        { index: 1, top: 1008, height: 1000 },
+      ],
+      1350,
+      1,
+    );
+
+    const restored = restoreScrollModeAnchor(
+      [
+        { index: 0, top: 0, height: 900 },
+        { index: 1, top: 908, height: 900 },
+      ],
+      anchor,
+      5000,
+    );
+
+    expect(restored).toBeCloseTo(1215.8);
+    expect(restored).not.toBe(908);
+  });
+
+  it('falls back to the previous scrollTop when the anchor page disappears', () => {
+    const restored = restoreScrollModeAnchor(
+      [{ index: 0, top: 0, height: 900 }],
+      { index: 1, fraction: 0.4, scrollTop: 1350 },
+      1200,
+    );
+
+    expect(restored).toBe(1200);
+  });
+});