switch to flask_login, finish login implementation, add basic home page

app/models/user.py

@@ -0,0 +1,16 @@
+from typing import Optional
+from app.db import get_db
+from flask_login import UserMixin
+
+class User(UserMixin):
+    def __init__(self, id: int, username: str ):
+        self.id: int = id
+        self.username: str = username
+
+    @staticmethod
+    def get(user_id: int | str) -> Optional["User"]:
+        db = get_db()
+        user = db.execute("SELECT * FROM users WHERE id = ?", (user_id,)).fetchone()
+        if user:
+            return User(user["id"], user["username"])
+        return None