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

frontend/src/locales/en.json

@@ -1,6 +1,7 @@
 {
 	"navbar": {
 		"collections": "Collections",
+		"chat": "Travel Agent",
 		"map": "Map",
 		"users": "Users",
 		"search": "Search",
@@ -31,6 +32,20 @@
 		},
 		"navigation": "Navigation"
 	},
+	"chat": {
+		"title": "Travel Agent",
+		"conversations": "Conversations",
+		"new_conversation": "New Conversation",
+		"untitled": "Untitled Conversation",
+		"no_conversations": "No conversations yet. Start chatting!",
+		"welcome_title": "Welcome to the Travel Agent",
+		"welcome_message": "I can help you discover amazing places, plan your trips, and organize your itineraries. What would you like to explore?",
+		"input_placeholder": "Ask me about travel destinations, plan a trip, or get recommendations...",
+		"send": "Send",
+		"delete_conversation": "Delete Conversation",
+		"connection_error": "Connection error. Please try again.",
+		"no_api_key": "No API key found. Please add one in Settings."
+	},
 	"about": {
 		"about": "About",
 		"license": "Licensed under the GPL-3.0 License.",