refactor: refactor for modern layout

- split frontend and backend.
- update backend with modern Python dev strategies.

backend/coconut-leaf.py

@@ -0,0 +1,76 @@
+import sys
+import logging
+from argparse import ArgumentParser
+from typing import cast
+from pathlib import Path
+import server
+import config
+import utils
+import database
+
+
+def GetUsernamePassword():
+    print("What is the first username of this calendar system?")
+    cache = input()
+    while not utils.IsValidUsername(cache):
+        print("Sorry, invalid data. Please try again.")
+        cache = input()
+    username = cache
+
+    print("Input this user password:")
+    cache = input()
+    while not utils.IsValidPassword(cache):
+        print("Sorry, invalid data. Please try again.")
+        cache = input()
+    password = cache
+
+    return (username, password)
+
+
+if __name__ == "__main__":
+    print("Coconut-leaf")
+    print("A self-host, multi-account calendar system.")
+    print("Project: https://github.com/yyc12345/coconut-leaf")
+    print("===================")
+
+    # Receive arguments
+    parser = ArgumentParser(description="Coconut-leaf")
+    parser.add_argument(
+        "-c",
+        "--config",
+        required=True,
+        type=Path,
+        action="store",
+        metavar="CONFIG_TOML",
+        dest="config",
+        help="The configuration file for coconut-leaf",
+    )
+    parser.add_argument(
+        "-i",
+        "--init",
+        action="store_true",
+        dest="init",
+        help="Set for initialize the calendar system",
+    )
+    args = parser.parse_args()
+
+    # Load config file
+    try:
+        config.setup_config(cast(Path, args.config))
+    except Exception as e:
+        print(f"Error loading config file: {e}")
+        sys.exit(1)
+
+    # Setup logging level
+    logging_level = logging.DEBUG if config.get_config().others.debug else logging.INFO
+    logging.basicConfig(format='[%(levelname)s] %(message)s', level=logging_level)
+
+    # Initialize the calendar system if needed
+    if cast(bool, args.init):
+        gotten_data = GetUsernamePassword()
+        calendar = database.CalendarDatabase()
+        calendar.init(*gotten_data)
+        calendar.close()
+
+    logging.info("Staring server...")
+    server.run()