refactor: extract magic number 4096 as named BUFFER_SIZE constant

Replace the hardcoded buffer size literal 4096 with a named constant BUFFER_SIZE
to improve code readability and maintainability. The constant is defined at the
module level and used in all buffer allocation sites.

src/shadowssh/app.py

@@ -4,6 +4,8 @@ from app.app import Application as BaseApplication
 
 from shadowssh import log
 
+BUFFER_SIZE = 4096
+
 
 class Application(BaseApplication):
 
@@ -29,7 +31,7 @@ class Application(BaseApplication):
 
     async def stream(self, reader, writer, port):
         while True:
-            chunk = await reader.read(64)
+            chunk = await reader.read(BUFFER_SIZE)
             if not chunk:
                 writer.close()
                 log.info("Forward connection closed.")