Security, code quality and frontend improvements

- Move SECRET_KEY out of docker-compose into .env (env_file), add .env.example
- Add flask-limiter with 10 req/min on login route; introduce util/limiter.py
- Replace direct mysql.connector.connect() calls with MySQLConnectionPool via util/db_pool.py
- Fix deprecated datetime.utcnow() -> datetime.now(timezone.utc) in auth/login.py
- Remove dead /api/scripts 410 route from admin.py
- Add MD5 security warning in Md5Tool.jsx
- Add ErrorBoundary component and wrap App.jsx
- Expand README with setup guide, screenshot and project structure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/ErrorBoundary.jsx

@@ -0,0 +1,33 @@
+import { Component } from 'react';
+
+class ErrorBoundary extends Component {
+  constructor(props) {
+    super(props);
+    this.state = { hasError: false, error: null };
+  }
+
+  static getDerivedStateFromError(error) {
+    return { hasError: true, error };
+  }
+
+  componentDidCatch(error, info) {
+    console.error('ErrorBoundary caught:', error, info);
+  }
+
+  render() {
+    if (this.state.hasError) {
+      return (
+        <div style={{ padding: '2rem', textAlign: 'center' }}>
+          <h2>Ein unerwarteter Fehler ist aufgetreten.</h2>
+          <p style={{ opacity: 0.7 }}>{this.state.error?.message}</p>
+          <button onClick={() => this.setState({ hasError: false, error: null })}>
+            Erneut versuchen
+          </button>
+        </div>
+      );
+    }
+    return this.props.children;
+  }
+}
+
+export default ErrorBoundary;

frontend/src/components/Md5Tool.jsx

@@ -18,11 +18,16 @@ function Md5Tool() {
   return (
     <div className="main-content">
       <h2>MD5 Hasher</h2>
+      <p style={{ color: '#e07b00', fontSize: '0.875rem', marginBottom: '0.75rem' }}>
+        <strong>Sicherheitshinweis:</strong> MD5 ist kryptografisch unsicher und darf
+        nicht zum Hashen von Passwörtern verwendet werden. Für Passwörter bitte
+        bcrypt, Argon2 oder scrypt einsetzen.
+      </p>
       <input
         type="text"
         value={input}
         onChange={(e) => setInput(e.target.value)}
-        placeholder="Gib ein Passwort ein"
+        placeholder="Eingabe"
       />
       <button onClick={hashPassword}>Hash berechnen</button>
       {result && <p><strong>MD5:</strong> {result}</p>}