Add error handlers for 404 and 500 pages

Register Flask error handlers to render the error templates.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/smarttools/web/app.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 import os
 import secrets
 
-from flask import Flask, session
+from flask import Flask, render_template, session
 
 from smarttools.registry import app as registry_app
 
@@ -50,6 +50,15 @@ def create_web_app() -> Flask:
     app.add_url_rule("/sitemap.xml", endpoint="web.sitemap", view_func=sitemap_response)
     app.add_url_rule("/robots.txt", endpoint="web.robots", view_func=robots_txt)
 
+    # Error handlers
+    @app.errorhandler(404)
+    def not_found_error(error):
+        return render_template("errors/404.html"), 404
+
+    @app.errorhandler(500)
+    def internal_error(error):
+        return render_template("errors/500.html"), 500
+
     return app