feat: 放大标题并调整标题框

- 标题框左右各留 2 格，高度改为 4 行
- 标题字号 1.6em 放大到 1.8em
- PDF 默认输出恢复为 dist/封面demo.pdf，移除根目录旧 PDF

scripts/export_pdf.py

@@ -5,7 +5,7 @@
 
 用法：
     uv run python scripts/export_pdf.py
-    uv run python scripts/export_pdf.py --em 16 -o 封面demo-16.pdf
+    uv run python scripts/export_pdf.py --em 16 -o dist/封面demo-16.pdf
 """
 
 from __future__ import annotations
@@ -138,8 +138,8 @@ def main() -> None:
         "-o",
         "--out",
         type=pathlib.Path,
-        default=ROOT / "封面demo.pdf",
-        help="输出文件路径，默认 封面demo.pdf",
+        default=ROOT / "dist" / "封面demo.pdf",
+        help="输出文件路径，默认 dist/封面demo.pdf",
     )
     args = parser.parse_args()
     if args.em <= 0: