feat: add Export to PDF button for expense reports Closes #12

[adithyen]

📌 Description

Brief description of what this PR does.
Adds an "Export to PDF" button next to the monthly total that generates a clean, professional PDF report of the user's expense data. Uses jsPDF + jspdf-autotable (loaded via CDN) for client-side PDF generation — no server-side changes needed.

🔗 Related Issue

Closes #12

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🛠 Changes Made

• Added jsPDF and jspdf-autotable CDN scripts to index.html
• Added "📄 Export to PDF" button next to the Selected Month Total
• Added exportToPDF() function in script.js that generates a PDF with:
  • Report title and generation date
  • Active filter info (month, category)
  • Expense table with all currently displayed data
  • Category-wise summary table
  • Budget status (exceeded / approaching / within limit)
  • Page footers with page numbers
• Added .export-btn styling in style.css (blue button, dark mode support)
• PDF filename is dynamic: expenses_YYYY-MM.pdf when filtered by month, otherwise expenses_report.pdf

📷 Screenshots (if applicable)

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✅ Checklist

• I have tested my changes
• My code follows project guidelines
• I have linked the related issue