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/urls.py

@@ -0,0 +1,11 @@
+from django.urls import include, path
+from rest_framework.routers import DefaultRouter
+
+from .views import ChatViewSet
+
+router = DefaultRouter()
+router.register(r"conversations", ChatViewSet, basename="chat-conversation")
+
+urlpatterns = [
+    path("", include(router.urls)),
+]