feat(chat): add LLM-powered travel agent with multi-provider support

Implement a full chat-based travel agent using LiteLLM for multi-provider
LLM support (OpenAI, Anthropic, Gemini, Ollama, Groq, Mistral, etc.).

Backend:
- New 'chat' Django app with ChatConversation and ChatMessage models
- Streaming SSE endpoint via StreamingHttpResponse
- 5 agent tools: search_places, list_trips, get_trip_details,
  add_to_itinerary, get_weather
- LiteLLM client wrapper with per-user API key retrieval
- System prompt with user preference context injection

Frontend:
- New /chat route with full-page chat UI (DaisyUI + Tailwind)
- Collapsible conversation sidebar with CRUD
- SSE streaming response display with tool call visualization
- Provider selector dropdown
- SSE proxy fix to stream text/event-stream without buffering
- Navbar link and i18n keys

backend/server/chat/serializers.py

@@ -0,0 +1,25 @@
+from rest_framework import serializers
+
+from .models import ChatConversation, ChatMessage
+
+
+class ChatMessageSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = ChatMessage
+        fields = [
+            "id",
+            "role",
+            "content",
+            "tool_calls",
+            "tool_call_id",
+            "name",
+            "created_at",
+        ]
+
+
+class ChatConversationSerializer(serializers.ModelSerializer):
+    messages = ChatMessageSerializer(many=True, read_only=True)
+
+    class Meta:
+        model = ChatConversation
+        fields = ["id", "title", "created_at", "updated_at", "messages"]