Fix Docker build by splitting client-safe gua utils from fs-based content loader.

Client components were importing zhouyi.ts which pulled in fs/promises; move pure helpers to gua-utils.ts and mark zhouyi.ts server-only.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/content/gua-utils.ts

@@ -0,0 +1,58 @@
+/** 客户端/服务端均可用的卦名工具（不含 fs） */
+
+export type LearnVariant = "traditional" | "simplified";
+
+export function guaNumFromMark(guaMark: string): string {
+  return guaMark.split(".")[0];
+}
+
+export function getGuaNumber(guaMark: string): number {
+  return parseInt(guaMark.split(".")[0], 10);
+}
+
+export function getGuaName(guaMark: string): string {
+  return guaMark.split(".").slice(1).join(".");
+}
+
+export function stripFrontmatter(content: string): string {
+  if (!content.startsWith("---")) {
+    return content;
+  }
+  const end = content.indexOf("---", 3);
+  if (end === -1) {
+    return content;
+  }
+  return content.slice(end + 3).trimStart();
+}
+
+export function extractZhangMingRen(guaDetail: string): string | undefined {
+  return guaDetail
+    .match(/(\*\*台灣張銘仁[\s\S]*?)(?=周易第\d+卦)/)?.[1]
+    ?.replaceAll("\n\n", "\n");
+}
+
+export function extractChangeDetails(
+  guaDetail: string,
+  guaChange: string,
+  guaTitle: string,
+): string[] {
+  const changeList: string[] = [];
+  if (guaChange === "无变爻") {
+    return changeList;
+  }
+
+  guaChange
+    .split(":")[1]
+    .trim()
+    .split(",")
+    .forEach((change) => {
+      const detail = guaDetail
+        .match(`(\\*\\*${change}變卦[\\s\\S]*?)(?=${guaTitle}|$)`)?.[1]
+        ?.replaceAll("\n\n", "\n");
+      if (detail) {
+        changeList.push(detail.trim());
+      }
+    });
+
+  return changeList;
+}