feat: Implements a Local City, County & School District Income Tax Withholding and Reciprocity Engine (#2062)

[Aakif-Kohari]

Description

This PR implements a Local City, County & School District Income Tax Withholding & Reciprocity Engine. It introduces schemas to track municipal jurisdictions, commuter tax rules, employee tax certificates, and YTD local wage accumulators. The engine evaluates residency vs. work-location matrices, applies commuter tax credits (ensuring employees aren't double-taxed by home and work municipalities under frameworks like PA Act 32), and enforces local-specific wage bases. A "Jurisdiction Conflict Guardrail" automatically flags employees whose addresses trigger conflicting local tax claims or missing certificates.

Fixes#2062

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Changes Made

• backend/src/constants/localTax.constants.js (New): Tax types, reciprocity frameworks, and municipality codes.
• backend/src/models/localTax.model.js (New): Schemas for LocalTaxJurisdiction, CommuterTaxRule, EmployeeTaxCertificate, and LocalTaxLedger.
• backend/src/utils/localTaxEngine.utils.js (New): Engine for withholding calculations, commuter credits, and conflict guardrails.
• backend/src/controllers/localTax.controller.js (New): Endpoints for jurisdiction management, certificate submission, and payroll processing.
• backend/src/routes/localTax.routes.js (New): Route definitions.
• frontend/src/components/JurisdictionConflictAlert.jsx (New): Reusable alert for missing certificates and double-taxation risks.
• frontend/src/pages/LocalTaxDashboard.jsx (New): Main dashboard for managing jurisdictions, rules, and YTD liabilities.

How Has This Been Tested?

• Tested locally on frontend
• Tested locally on backend

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

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[Aakif-Kohari]

Please try to add many Bonus XP tags (good-issue, good-pr, good-ui, good-backend) before merge.🙏