Add UK Local Authority Selection

[anth-volk]

Fixes #517

Summary

Adds support for selecting UK local authorities as a geography option, enabling users to run policy simulations scoped to any of the ~360 local authorities across England, Scotland, Wales, and Northern Ireland.

This change complements policyengine-api#2992 which adds local authority metadata and breakdown calculations to the API.

Changes

• Add LOCAL_AUTHORITY to UK_REGION_TYPES constant
• Add getUKLocalAuthorities() filter function in regionStrategies.ts
• Create dedicated selector components following US pattern:
  • UKCountrySelector.tsx
  • UKConstituencySelector.tsx
  • UKLocalAuthoritySelector.tsx
• Update UKGeographicOptions.tsx to use new selector components
• Wire up local authority options in PopulationScopeView.tsx
• Add local authority prefix handling in geographyUtils.ts
• Add test fixtures and unit tests

Test Plan

• Verify local authority option appears in UK geography selection
• Verify local authority dropdown is searchable with ~360 options
• Verify selecting a local authority creates correct geography object
• Run existing tests to ensure no regressions

🤖 Generated with Claude Code

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