feat: Implement main application shell with navigation and color scheme toggle

- Added Appshell component with responsive navbar and main content area
- Integrated ColorSchemeToggle for light/dark mode switching
- Created Welcome component with styled title and introductory text
- Developed ChatPage for LLM interaction with WebSocket support
- Implemented Biblioteca for managing notes with rich text editor
- Added LoginPage for user authentication with error handling
- Introduced MessageList and MessageBubble components for chat messages
- Styled components with CSS modules for consistent design

domains/ConexionSql/Postgres_conexion.py

@@ -0,0 +1,58 @@
+from datetime import datetime, timezone
+from sqlalchemy import create_engine, text
+from sqlalchemy.exc import SQLAlchemyError
+from sqlalchemy.orm import sessionmaker, Session
+from sqlalchemy.engine import Engine
+
+from domains.ConexionSql.Base_conexion import ConexionBase
+from domains.Credenciales.postgres_credencial import PostgresCredencial
+
+class PostgresConexion(ConexionBase):
+    def __init__(self, *args, **kwargs):
+        self.estado = "pendiente"
+        self.timestamp = datetime.now(timezone.utc)
+
+        if args and isinstance(args[0], PostgresCredencial):
+            credencial = args[0]
+            self.host = credencial.host
+            self.port = credencial.port
+            self.dbname = credencial.dbname
+            self.user = credencial.user
+            self.password = credencial.password
+            uri = credencial.get_uri()
+        else:
+            self.user = kwargs.get("user")
+            self.password = kwargs.get("password")
+            self.host = kwargs.get("host")
+            self.port = kwargs.get("port", 5432)
+            self.dbname = kwargs.get("db") or kwargs.get("dbname")
+            uri = f"postgresql://{self.user}:{self.password}@{self.host}:{self.port}/{self.dbname}"
+
+        self._engine: Engine = create_engine(uri)
+        self._Session = sessionmaker(bind=self._engine)
+
+        # ✅ INICIALIZAR LA SESIÓN AQUÍ
+        self._session_instance: Session | None = None
+
+    def get_session(self) -> Session:
+        if self._session_instance is None:
+            self._session_instance = self._Session()
+        return self._session_instance
+
+    def get_engine(self) -> Engine:
+        return self._engine
+
+    def probar_conexion(self) -> bool:
+        try:
+            with self._engine.connect() as connection:
+                connection.execute(text("SELECT 1"))
+            self.estado = "exito"
+            return True
+        except SQLAlchemyError:
+            self.estado = "fallo"
+            return False
+
+    def close(self):
+        if self._session_instance is not None:
+            self._session_instance.close()
+            self._session_instance = None