feat: typing indicator mit agent-name und random phrases

- typing-phrases.js in index.html eingebunden
- startTypingIndicator() / stopTypingIndicator() in chat.js
- Agent-Name via setCurrentAgentName() aus chat-page.js gesetzt
- Phrase wechselt alle 3 Sekunden zufällig
- Cleanup bei destroy() und finally-Block

wwwroot/index.html

@@ -616,6 +616,7 @@
 <script src="js/reply.js"></script>
 <script src="js/files.js"></script>
 <script src="js/file-explorer.js"></script>
+<script src="js/typing-phrases.js"></script>
 <script src="js/chat.js"></script>
 <script src="js/history.js"></script>
 <script src="js/archive.js"></script>