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/bun.lock

@@ -6,6 +6,7 @@
       "name": "voyage-frontend",
       "dependencies": {
         "@lukulent/svelte-umami": "^0.0.3",
+        "@mdi/js": "^7.4.47",
         "dompurify": "^3.3.2",
         "emoji-picker-element": "^1.29.1",
         "gsap": "^3.14.2",
@@ -47,7 +48,6 @@
     },
   },
   "overrides": {
-    "cookie@<0.7.0": ">=0.7.0",
     "devalue": "^5.6.2",
     "esbuild": "^0.26.0",
     "minimatch": "^10.2.1",
@@ -170,6 +170,8 @@
 
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+    "@mdi/js": ["@mdi/js@7.4.47", "", {}, "sha512-KPnNOtm5i2pMabqZxpUz7iQf+mfrYZyKCZ8QNz85czgEt7cuHcGorWfdzUMWYA0SD+a6Hn4FmJ+YhzzzjkTZrQ=="],
+
     "@nodelib/fs.scandir": ["@nodelib/fs.scandir@2.1.5", "", { "dependencies": { "@nodelib/fs.stat": "2.0.5", "run-parallel": "^1.1.9" } }, "sha512-vq24Bq3ym5HEQm2NKCr3yXDwjc7vTsEThRDnkp2DK9p1uqLR+DHurm/NOTo0KG7HYHU7eppKZj3MyqYuMBf62g=="],
 
     "@nodelib/fs.stat": ["@nodelib/fs.stat@2.0.5", "", {}, "sha512-RkhPPp2zrqDAQA/2jNhnztcPAlv64XdhIp7a7454A5ovI7Bukxgt7MX7udwAu3zg1DcpPU0rz3VV1SeaqvY4+A=="],