feat: persist chat session across page reloads

Session ID saved to localStorage (schaeffler-chat-session).
On mount, restores the last session and loads messages from DB.
"New Chat" clears the stored session.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/chat/ChatPanel.tsx

@@ -20,9 +20,20 @@ interface ChatPanelProps {
 
 export default function ChatPanel({ open, onClose, contextType, contextId }: ChatPanelProps) {
   const [messages, setMessages] = useState<ChatMessage[]>([])
-  const [sessionId, setSessionId] = useState<string | undefined>()
+  const [sessionId, setSessionId] = useState<string | undefined>(() => {
+    // Restore last session from localStorage
+    try { return localStorage.getItem('schaeffler-chat-session') || undefined } catch { return undefined }
+  })
   const [input, setInput] = useState('')
   const [showSessions, setShowSessions] = useState(false)
+
+  // Persist sessionId to localStorage
+  useEffect(() => {
+    try {
+      if (sessionId) localStorage.setItem('schaeffler-chat-session', sessionId)
+      else localStorage.removeItem('schaeffler-chat-session')
+    } catch { /* ignore */ }
+  }, [sessionId])
   const messagesEndRef = useRef<HTMLDivElement>(null)
   const inputRef = useRef<HTMLInputElement>(null)
   const queryClient = useQueryClient()