fix: add CJK font support for PDF conversion

When converting documents containing Chinese, Japanese, or Korean
characters to PDF via pandoc/xelatex, the output was blank or missing
glyphs because no CJK-capable font was installed or configured.

Changes:
- Dockerfile: Add fonts-noto-cjk and texlive-lang-chinese packages
- pandoc.ts: Pass -V CJKmainfont=Noto Sans CJK SC to pandoc when
  converting to pdf/latex, so xelatex uses a CJK-capable font
- pandoc.test.ts: Add 3 tests verifying CJK font argument is present
  for pdf/latex output and absent for other formats

Fixes #547

Dockerfile

@@ -73,9 +73,11 @@ RUN apt-get update && apt-get install -y \
   resvg \
   texlive \
   texlive-fonts-recommended \
+  texlive-lang-chinese \
   texlive-latex-extra \
   texlive-latex-recommended \
   texlive-xetex \
+  fonts-noto-cjk \
   python3 \
   python3-pip \
   pipx \

src/converters/pandoc.ts

@@ -136,6 +136,11 @@ export function convert(
 
   if (xelatex.includes(convertTo)) {
     args.push("--pdf-engine=xelatex");
+    // Specify a CJK-capable main font so that documents containing Chinese,
+    // Japanese or Korean characters render correctly instead of producing
+    // blank/missing glyphs. "Noto Sans CJK SC" is provided by the
+    // fonts-noto-cjk package installed in the Dockerfile.
+    args.push("-V", "CJKmainfont=Noto Sans CJK SC");
   }
 
   args.push(filePath);

tests/converters/pandoc.test.ts

@@ -57,6 +57,47 @@ describe("convert", () => {
     expect(calledArgs[1]).toContain("output.pdf");
   });
 
+  test("should add CJK mainfont argument for pdf output", async () => {
+    let calledArgs: Parameters<ExecFileFn> = ["", [], () => {}];
+    mockExecFile = (cmd, args, callback) => {
+      calledArgs = [cmd, args, callback];
+      callback(null, "output-data", "");
+    };
+
+    await convert("input.md", "markdown", "pdf", "output.pdf", undefined, mockExecFile);
+
+    const cjkFontIndex = calledArgs[1].indexOf("-V");
+    expect(cjkFontIndex).toBeGreaterThan(-1);
+    expect(calledArgs[1][cjkFontIndex + 1]).toBe("CJKmainfont=Noto Sans CJK SC");
+  });
+
+  test("should add CJK mainfont argument for latex output", async () => {
+    let calledArgs: Parameters<ExecFileFn> = ["", [], () => {}];
+    mockExecFile = (cmd, args, callback) => {
+      calledArgs = [cmd, args, callback];
+      callback(null, "output-data", "");
+    };
+
+    await convert("input.md", "markdown", "latex", "output.tex", undefined, mockExecFile);
+
+    expect(calledArgs[1][0]).toBe("--pdf-engine=xelatex");
+    const cjkFontIndex = calledArgs[1].indexOf("-V");
+    expect(cjkFontIndex).toBeGreaterThan(-1);
+    expect(calledArgs[1][cjkFontIndex + 1]).toBe("CJKmainfont=Noto Sans CJK SC");
+  });
+
+  test("should not add CJK mainfont for non-pdf/latex output", async () => {
+    let calledArgs: Parameters<ExecFileFn> = ["", [], () => {}];
+    mockExecFile = (cmd, args, callback) => {
+      calledArgs = [cmd, args, callback];
+      callback(null, "output-data", "");
+    };
+
+    await convert("input.md", "markdown", "html", "output.html", undefined, mockExecFile);
+
+    expect(calledArgs[1].some((arg) => arg.startsWith("CJKmainfont"))).toBe(false);
+  });
+
   test("should reject if execFile returns an error", async () => {
     mockExecFile = (cmd, args, callback) => callback(new Error("fail"), "", "");
     await expect(