feat(newsletter): Add subscription confirmation email functionality

- Implemented `send_subscription_confirmation` function to send a confirmation email upon subscription.
- Added a default HTML template for the confirmation email in settings.
- Updated the newsletter management page to handle subscription and unsubscription actions.
- Created new templates for embedding contact and newsletter forms.
- Added styles for the new templates and unified CSS styles across the application.
- Updated tests to reflect changes in the newsletter management API endpoints.

server/routes/embed.py

@@ -0,0 +1,26 @@
+"""Embeddable forms routes."""
+from __future__ import annotations
+
+from flask import Blueprint, render_template, send_from_directory
+import os
+
+bp = Blueprint("embed", __name__)
+
+
+@bp.route("/embed/contact", methods=["GET"])
+def contact_form():
+    """Serve the embeddable contact form."""
+    return render_template("embed_contact.html")
+
+
+@bp.route("/embed/newsletter", methods=["GET"])
+def newsletter_form():
+    """Serve the embeddable newsletter subscription form."""
+    return render_template("embed_newsletter.html")
+
+
+@bp.route("/static/css/styles.css", methods=["GET"])
+def serve_css():
+    """Serve the unified CSS file."""
+    static_dir = os.path.join(os.path.dirname(__file__), '..', '..', 'static')
+    return send_from_directory(static_dir, 'css/styles.css')