Generate yearly tax summary

[rappm]

Goal

Generate a selected-year tax summary covering dividends, capital gains, Quellensteuer, and Freistellungsauftrag utilization.

Source

• FR-T4 in deliverables/PROBLEM_STATEMENT.md

Change

• Aggregate tax-relevant events by selected tax year.
• Render a summary view and export-ready data where feasible.

Done When

• A user can select a tax year.
• The summary includes dividends, capital gains, Quellensteuer, and Freistellungsauftrag utilization.
• The summary handles years with no data clearly.