feat(latex): 新增 LaTeX 排版系统，从 Markdown 生成 ctexbook

- 新增 officefile/latex/ 目录，包含 main.tex、preamble.tex 及 14 个章节 + 10 个附录 tex 文件
- md→tex 转换流程：pandoc 转换 + _postprocess.py 后处理（去编号、修破折号、清标签）
- 修复 5 个 md 源文件的标题层级（H1→H2 降级，统一层级结构）
- 配置 VS Code LaTeX Workshop（xelatex + ctexbook 编译链）
- 新增 VS Code workspace 和 .gitignore（排除 LaTeX 编译产物）

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.vscode/settings.json

@@ -0,0 +1,33 @@
+{
+  "latex-workshop.latex.tools": [
+    {
+      "name": "xelatex",
+      "command": "D:/Software/texlive/2026/bin/windows/xelatex.exe",
+      "args": [
+        "-synctex=1",
+        "-interaction=nonstopmode",
+        "-file-line-error",
+        "%DOC%"
+      ],
+      "env": {
+        "FONTCONFIG_PATH": "D:/Software/texlive/2026/texmf-var/fonts/conf"
+      }
+    },
+    {
+      "name": "bibtex",
+      "command": "D:/Software/texlive/2026/bin/windows/bibtex.exe",
+      "args": ["%DOCFILE%"]
+    }
+  ],
+  "latex-workshop.latex.recipes": [
+    {
+      "name": "xelatex -> bibtex -> xelatex * 2",
+      "tools": ["xelatex", "bibtex", "xelatex", "xelatex"]
+    },
+    {
+      "name": "xelatex",
+      "tools": ["xelatex"]
+    }
+  ],
+  "latex-workshop.latex.outDir": "%DIR%"
+}