Add cash flow transaction drill-down

Summary

Added a month-aware Cash Flow detail page from the dashboard that lists positive cash and settlement outflows, groups them by Essential, Flexible, and Daily, shows section subtotals, and supports accessible expand and collapse controls.

Decisions

• Reused the existing monthly transaction endpoint and filtered the returned rows to spending sections with cash or settlement kinds and positive amounts
• Ordered sections as Essential, Flexible, then Daily and kept each section expanded by default with its subtotal visible when collapsed