Add fee-adjusted APY controls

[Sikkra]

Summary

• add fee-drag APR math using rebalances/year x 0.25 XLM x XLM price x leverage / equity
• subtract annualized fee drag from displayed net APY across preview, chart, position, portfolio, overview, and vault views
• add a per-wallet/per-network persisted rebalances/year control with a documented 12/year default
• fill demo reserve rate config so the preview path can exercise projected-rate math without runtime errors

Closes #29

Verification

• npm run build
• Playwright/Edge smoke on local Vite port 5207: default 12/year, scoped default/demo-user persistence, visible fee drag -1.20% APR, tooltip formula/default, no browser console or page errors
• git diff --check