fix: use apiRequest and API_CONFIG in usePortfolio hook

[TCROWN10]

Summary

• Route usePortfolio through apiRequest and API_CONFIG so portfolio fetch/rebalance hit the configured API origin instead of relative /api/... paths
• Pick up retry (3 attempts with backoff), 15s timeout, and JSON error parsing from the shared client
• Keep setPortfolio(data.portfolio) aligned with { success: true, portfolio } from GET /api/portfolio/:id

Closes #66

Test plan

• Frontend npm run build (tsc && vite build) succeeds
• Loading a portfolio with VITE_API_URL set to a remote origin calls that host, not the frontend origin
• Failed network requests surface hook error and retry per apiRequest
• Successful rebalance refreshes portfolio from parsed JSON