feat(frontend): cbz page/spread view math (TDD)

frontend/src/lib/cbzPages.ts

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+export type PageMode = "page" | "spread";
+
+export function viewCount(count: number, mode: PageMode): number {
+  return mode === "page" ? count : Math.ceil(count / 2);
+}
+export function pageIndexToView(i: number, mode: PageMode): number {
+  return mode === "page" ? i : Math.floor(i / 2);
+}
+export function viewToPageIndex(v: number, mode: PageMode): number {
+  return mode === "page" ? v : v * 2;
+}
+/** 一个 spread 视图要渲染的页,按显示顺序（LTR=[左,右];RTL=[右,左] 即先读页在前）。 */
+export function spreadPages(first: number, count: number, rtl: boolean): number[] {
+  const pages = [first, ...(first + 1 < count ? [first + 1] : [])];
+  return rtl ? [...pages].reverse() : pages;
+}
+/** 翻一屏/页；rtl 时方向翻转；越界钳制。 */
+export function turnView(v: number, d: number, rtl: boolean, views: number): number {
+  const step = rtl ? -d : d;
+  return Math.max(0, Math.min(views - 1, v + step));
+}

frontend/test/cbzPages.test.ts

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+import { describe, expect, it } from "vitest";
+import { spreadPages, pageIndexToView, turnView, viewCount, viewToPageIndex } from "../src/lib/cbzPages";
+
+describe("cbzPages", () => {
+  it("page mode: one page per view", () => {
+    expect(viewCount(5, "page")).toBe(5);
+    expect(viewToPageIndex(2, "page")).toBe(2);
+    expect(pageIndexToView(2, "page")).toBe(2);
+  });
+  it("spread mode: pairs [0,1] [2,3] [4]", () => {
+    expect(viewCount(5, "spread")).toBe(3);
+    expect(pageIndexToView(3, "spread")).toBe(1);
+    expect(viewToPageIndex(1, "spread")).toBe(2);
+    expect(spreadPages(2, 5, false)).toEqual([2, 3]);
+    expect(spreadPages(4, 5, false)).toEqual([4]); // 落单页
+    expect(spreadPages(2, 5, true)).toEqual([3, 2]); // RTL 右页为先
+  });
+  it("turnView clamps and RTL flips direction", () => {
+    expect(turnView(0, 1, false, 3)).toBe(1);
+    expect(turnView(2, 1, false, 3)).toBe(2); // 右边界钳制
+    expect(turnView(0, -1, false, 3)).toBe(0); // 左边界钳制
+    expect(turnView(1, 1, true, 3)).toBe(0); // RTL：d=+1 变后退
+  });
+});