Expose free-money breakdown in dashboard API

Summary

Added nine monthly dashboard response fields covering spending sums, budgets, and remaining amounts for Essential, Subscriptions, and Daily. Updated API documentation and integration-test assertions, including the free-money formula. The targeted Go test command compiled successfully, but integration tests were skipped because PENNYWISE_TEST_DATABASE_URL was not configured. Refreshed the code knowledge graph and appended an app-story entry.

Decisions

• Preserved selected-month budgets from monthly_budgets rather than reverting to user_settings.
• Preserved cash-plus-credit section sums excluding settlements, and allowed negative remaining budgets without zero-clamping.