Fix CORS for local dev and update stats grid layout

Add localhost:3001 to allowed CORS origins to fix local Docker setup
where frontend is mapped to port 3001. Also update stats grid to fixed
3-column layout and reorder stat cards.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

btc-portfolio/backend/app/main.py

@@ -10,7 +10,7 @@ app = FastAPI(title="BTC Portfolio API")
 
 app.add_middleware(
     CORSMiddleware,
-    allow_origins=["http://localhost:3000"],
+    allow_origins=["http://localhost:3000", "http://localhost:3001"],
     allow_credentials=True,
     allow_methods=["*"],
     allow_headers=["*"],