修复 serve.py 未声明 charset 导致中文乱码

http.server 的 guess_type() 从不附带 charset，直接导航到 .cpp/.md 等
文本文件时浏览器猜编码猜错，中文注释显示乱码。Content-Type 统一补
charset=utf-8。

docs/teaching/serve.py

@@ -32,6 +32,16 @@ class ReuseAddrTCPServer(socketserver.TCPServer):
     allow_reuse_address = True
 
 
-with ReuseAddrTCPServer(("127.0.0.1", port), http.server.SimpleHTTPRequestHandler) as httpd:
+class Utf8RequestHandler(http.server.SimpleHTTPRequestHandler):
+    # http.server 的 guess_type() 从不附带 charset——中文源码/文档被当成
+    # 无编码信息的 text/plain 返回，浏览器直接导航时会猜错编码变成乱码。
+    def guess_type(self, path):
+        ctype = super().guess_type(path)
+        if ctype.startswith("text/") and "charset=" not in ctype:
+            ctype += "; charset=utf-8"
+        return ctype
+
+
+with ReuseAddrTCPServer(("127.0.0.1", port), Utf8RequestHandler) as httpd:
     print(f"Serving {repo_root} at http://localhost:{port}/docs/teaching/slides/index.html")
     httpd.serve_forever()