Add UK Gift Aid support - Parent Issue

[MaxGhenis]

Summary

This is the parent issue tracking UK Gift Aid support for GiveCalc. The UK has a different charitable giving tax system than the US:

UK Charitable Giving Tax Benefits

• Gift Aid: Charities can claim 25p for every £1 donated by UK taxpayers (basic rate relief at 20%)
• Higher rate relief: Higher/additional rate taxpayers can claim back the difference between their tax rate and basic rate (20% additional for 40% taxpayers, 25% additional for 45% taxpayers)
• Charitable Investment Gifts: Gifts of qualifying investments or property to charities
• Covenanted Payments: Historical mechanism for charitable donations

Implementation Tasks

The implementation is broken into the following sub-issues:

1. Add UK Gift Aid support - Parent Issue #59 - Add policyengine-uk dependency
2. Add policyengine-uk dependency #60 - Implement UK situation builder (givecalc package)
3. Implement UK situation builder #61 - Implement UK tax calculations (givecalc package)
4. Implement UK tax calculations #62 - Add UK API endpoints
5. Add UK API endpoints #63 - Update frontend for UK/US country selector

Technical Approach

• Create a parallel UK module structure under givecalc/uk/
• Use policyengine_uk for tax calculations
• UK uses gift_aid instead of charitable_cash_donations
• UK entities: Person → BenUnit → Household (vs US: Person → TaxUnit → Household)
• Show both donor tax savings AND charity Gift Aid reclaim

Key Differences from US

Aspect	US	UK
Main donation variable	charitable_cash_donations	gift_aid
Entity hierarchy	TaxUnit	BenUnit
State/region handling	50+ states	4 nations (England, Scotland, Wales, NI)
Tax relief mechanism	Itemized deduction	Gift Aid gross-up

Closes #58