feat: Add Tax Calculator & Financial Report Generator

[SatyamPandey-07]

• Add TaxProfile, TaxCategory, FinancialReport models
• Implement taxService with multi-country tax brackets (India/US)
• Add old vs new regime comparison for India
• Implement reportService for 7 report types
• Add pdfService for PDF export with PDFKit
• Create tax and reports REST API routes
• Add taxValidator middleware for request validation
• Create frontend tax-reports.js manager class
• Add tax-reports.html with calculator UI
• Update server.js with new routes
• Add comprehensive TAX_REPORTS.md documentation

Features:

• Tax calculation with surcharge and cess
• Deduction tracking (80C, 80D, etc.)
• Income statement, P&L, expense summary reports
• Monthly comparison and annual summary
• PDF download with professional formatting

closes #186

[vercel]

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[github-actions]

🎉 Thanks for the PR, @SatyamPandey-07!

We really appreciate you taking the time to contribute to OpenPlayground! 💙

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[Renu-code123]

@SatyamPandey-07 resolve conflict