Update configuration and enhance newsletter generation features

- Change WIKI_API_URL in .env.example and config to point to the new API endpoint.
- Set COOKIE_SECURE to false for local development in .env.example and config.
- Improve the newsletter generation logic to handle new and edited articles separately.
- Add display options for showing date, user, and category in the newsletter output.
- Enhance error handling for Wiki API requests and update templates for better user feedback.
- Update CSS for new UI elements and improve overall layout in dashboard and login pages.

app/services/smtp_sender.py

@@ -1,5 +1,6 @@
 import smtplib
 from datetime import datetime
+from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
 from typing import Sequence
 
@@ -40,7 +41,9 @@ def send_newsletter(
         _log(db, subject, recipients, "failed", "SMTP unvollständig konfiguriert.", scheduled)
         return
 
-    msg = MIMEText(html_content, "html", "utf-8")
+    msg = MIMEMultipart("alternative")
+    msg.attach(MIMEText(text_content, "plain", "utf-8"))
+    msg.attach(MIMEText(html_content, "html", "utf-8"))
     msg["Subject"] = subject
     msg["From"] = settings["from_email"]
     msg["To"] = ", ".join(recipients)