fix: correct capacity time-off handling and centralise quarter/revenue computations

[dholbach]

Summary

Review of all computation logic (revenue, sessions, capacity, tax, working days) for correctness, plus centralisation of duplicated calculation code.

Correctness fixes

• Weighted capacity ignored time off across capacity changes (capacity_helpers._calculate_weighted_capacity): for periods spanning the 2023-08-01 capacity change, the available_working_days parameter was unused, so vacation/time off never reduced usable capacity — utilisation percentages for such periods were understated. Now the per-segment weighted hours/week is applied to the time-off-adjusted available days.
• Quarterly tax revenue didn't match the year summary: tax_quarter_overview and TaxQuarterWidgetBuilder filtered strictly on paid_date__range, while the year summary uses build_paid_date_filter with an invoice-date fallback for paid invoices without a paid_date. Such invoices appeared in the yearly total but in no quarter. Quarters now use RevenueCalculator.build_paid_date_range_filter and sum exactly to the year total.
• count_working_days docstring example claimed 2025-11-15 is a Friday (it's a Saturday).

Centralisation

• New DateRangeHelper.get_quarter_range() / get_quarter_for_date() replace three independent quarter-boundary implementations (tax_views._quarter_date_range, TaxQuarterWidgetBuilder._quarter_of, and inline logic in calculate_quarter_trends — the latter carried a wrong year % 4 leap rule in dead code).
• practice_days.py: three identical _timeoff_dates() methods collapsed into one module-level _timeoff_dates_for_year().
• CapacityMonitoringWidgetBuilder._get_month_stats now reuses count_session_hours() and RevenueCalculator.get_month_revenue() instead of hand-rolled formulas.
• tax_views JSON error strings wrapped with gettext (P-039); German translations added (fixed two fuzzy-matched msgstrs from makemessages).

Verified as correct (no change needed)

• count_sessions / count_session_hours normalisation and group-size handling
• RevenueCalculator aggregations, apply_remainder_distribution
• Easter algorithm and Berlin holiday set in practice_days
• Entfernungspauschale (0.30/0.38 €/km split at 20 km) and Home-Office-Pauschale (6 €/day, 210-day cap)
• get_days_to_payment_trends DateField subtraction works correctly on PostgreSQL (exercised at runtime)

Tests

• New test_capacity_helpers.py: capacity period boundaries, weighted capacity with/without time off (regression), integration test across the capacity change
• Quarter helper tests in test_date_helpers.py
• Quarter/year revenue consistency regression tests in test_views_tax.py
• Full Django + JS suite passes

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