refactor: change backend as the pure API backend

backend/coconut-leaf.py

@@ -9,7 +9,7 @@ import utils
 import database
 
 
-def GetUsernamePassword():
+def GetUsernamePassword() -> tuple[str, str]:
     print("What is the first username of this calendar system?")
     cache = input()
     while not utils.IsValidUsername(cache):
@@ -27,14 +27,19 @@ def GetUsernamePassword():
     return (username, password)
 
 
+def SetLoggingStyle(level: int) -> None:
+    logging.basicConfig(format="[%(levelname)s] %(message)s", level=level)
+
+
 if __name__ == "__main__":
-    print("Coconut-leaf")
-    print("A self-host, multi-account calendar system.")
-    print("Project: https://github.com/yyc12345/coconut-leaf")
-    print("===================")
+    # Set as INFO level in default first,
+    # and we will change it once we load the configuration file.
+    SetLoggingStyle(logging.INFO)
 
     # Receive arguments
-    parser = ArgumentParser(description="Coconut-leaf")
+    parser = ArgumentParser(
+        description="The server of light, self-host and multi-account calendar system."
+    )
     parser.add_argument(
         "-c",
         "--config",
@@ -54,16 +59,22 @@ if __name__ == "__main__":
     )
     args = parser.parse_args()
 
+    # Show splash
+    logging.info("Coconut-leaf")
+    logging.info("A light, self-host and multi-account calendar system")
+    logging.info("Project: https://github.com/yyc12345/coconut-leaf")
+    logging.info("===================")
+
     # Load config file
     try:
         config.setup_config(cast(Path, args.config))
     except Exception as e:
-        print(f"Error loading config file: {e}")
+        logging.critical(f"Error loading config file: {e}")
         sys.exit(1)
 
-    # Setup logging level
+    # Change logging level again according to whether enable debug mode
     logging_level = logging.DEBUG if config.get_config().others.debug else logging.INFO
-    logging.basicConfig(format='[%(levelname)s] %(message)s', level=logging_level)
+    SetLoggingStyle(logging_level)
 
     # Initialize the calendar system if needed
     if cast(bool, args.init):