Implement CRA line 22215 deduction for CPP/QPP enhanced contributions

[xrendan]

Summary

Implements the CRA line 22215 deduction for enhanced CPP/QPP contributions. The enhanced portion of first-tier contributions (the rate above the pre-2019 base of 4.95% CPP / 5.40% QPP) plus 100% of CPP2/QPP2 is deducted from income before applying federal and provincial tax brackets — and before the Ontario surtax, which is computed from bracket tax.

Supersedes #274, re-implemented off current main (post #279 surtax fix) and simplified.

Changes

• Configs: Added baseRate to CPP/QPP configs for all years (2023–2026): 4.95% CPP, 5.40% QPP
• Calculator: New shared helper calculateEnhancedContributionPortion in calculators/cappedCalculator.ts; calculateWithConfig computes taxable income as income − deduction and applies it to both bracket calculations. The deduction total is exposed as cppQppEnhancedDeduction on DetailedTaxCalculation
• UI: Both tax cards on the visualizer show a "Total Taxable Income" section with a collapsible breakdown (gross income, CPP/QPP enhanced portion, CPP2/QPP2, taxable income); brackets are applied to taxable income

Differences from #274

• The duplicated enhanced-portion math (calculator + page) is extracted into one shared helper
• The negative cpp-qpp-enhanced-deduction TaxLineItem is dropped: nothing consumes it, and it would double-count in TaxBreakdownAccordion if that component were ever revived (the deduction is already baked into federalIncomeTax). The scalar on the calculation result remains
• The padded "reduces federal income tax" test now actually asserts both federal and provincial tax equal the bracket tax on income − deduction

Testing

• New calculator.deduction.test.ts validates the deduction amounts against hand-computed 2024 values ($838 at $80k for both CPP and QPP), low-income proportional scaling, and bracket wiring
• Full suite: 64 tests pass, lint clean
• Verified in the browser (Ontario and Quebec, 2025): collapsed summary reads "$100,000 gross income less $1,074 CPP/QPP enhanced deduction (line 22215)"; expanding shows the per-item breakdown with province-correct CPP/QPP labels