Fix AI chat conversation errors and history loading issues

- Fix CircularProgress import from correct MUI package
- Fix conversationId validation by omitting null values instead of sending them
- Fix conversations API Zod schema default values (string vs number mismatch)
- Remove debug logging from chat error handling
- AI chat conversations now work properly with authentication
- Chat history loading no longer fails with validation errors

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

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

components/chat/floating-chat.tsx

@@ -24,6 +24,7 @@ import {
   DialogTitle,
   DialogContent,
   DialogActions,
+  CircularProgress,
 } from '@mui/material'
 import {
   Chat,
@@ -39,7 +40,6 @@ import {
   History,
   Add,
   Delete,
-  CircularProgress,
   MoreVert,
   Edit,
 } from '@mui/icons-material'
@@ -351,7 +351,7 @@ export default function FloatingChat() {
         headers,
         body: JSON.stringify({
           message: inputMessage,
-          conversationId: activeConversationId,
+          ...(activeConversationId && { conversationId: activeConversationId }),
           history: messages.slice(-5), // Fallback for anonymous users
           locale: locale,
         }),