feat: implement employee charitable giving, corporate matching, and payroll deduction campaign engine (closes #2011)

[Aakif-Kohari]

Description

This PR implements an Employee Charitable Giving, Corporate Matching & Payroll Deduction Campaign Engine. It introduces schemas to track giving campaigns, employee pledges, and corporate match ledgers. The engine evaluates payroll deductions against pledge schedules, tracks YTD employee contributions, and calculates the corporate match up to the campaign cap. A "Campaign Cap Guardrail" automatically halts payroll deductions and corporate matching once the employee's pledge total or the corporate matching limit is reached. The frontend allows employees to submit pledges and CSR Admins to monitor participation and export disbursement files.

Fixes#2011

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/charitable.constants.js (New): Campaign statuses, matching rules, and deduction frequencies.
• backend/src/models/charitableGiving.model.js (New): Schemas for CharityOrganization, GivingCampaign, EmployeePledge, and CorporateMatchLedger.
• backend/src/utils/givingMatchingEngine.utils.js (New): Engine for period deductions, cap evaluations, and disbursement reports.
• backend/src/controllers/charitableGiving.controller.js (New): Endpoints for campaign management, pledge submission, and payroll processing.
• backend/src/routes/charitableGiving.routes.js (New): Route definitions.
• frontend/src/components/PledgeFormModal.jsx (New): Reusable modal for employees to submit new pledges.
• frontend/src/pages/CharitableGivingDashboard.jsx (New): Main dashboard for viewing campaigns and tracking personal pledges.

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.🙏