feat(chat): add guest chat mode for unauthenticated users

- Add POST /api/chat/guest endpoint (no auth required)
- Add proxyGuestChat() method using configurable LLM endpoint
- Add streamGuestChat() function to frontend chat API
- Modify useChat to fall back to guest mode on auth errors (403/401)
- Remove !user check from ChatInput disabled prop
- Configure guest LLM via env vars: GUEST_LLM_URL, GUEST_LLM_API_KEY, GUEST_LLM_MODEL
- Default guest LLM: http://10.1.1.42:11434/v1 (Ollama) with llama3.2 model

apps/web/src/components/chat/Chat.tsx

@@ -352,7 +352,7 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
         <div className="mx-auto max-w-4xl px-4 py-4 lg:px-8">
           <ChatInput
             onSend={handleSendMessage}
-            disabled={isChatLoading || !user}
+            disabled={isChatLoading}
             inputRef={inputRef}
             isStreaming={isStreaming}
             onStopStreaming={abortStream}