feat:add export to excel(.xlsx) functionality

[vaishnavi-rajesh]

📌 Description

This PR adds an "Export to Excel (.xlsx)" feature to the Expense Tracker. While PDF exports are great for summaries, this feature allows users to export their data in a structured spreadsheet format for deeper analysis (sorting, filtering, and custom calculations) in tools like Microsoft Excel or Google Sheets

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🔗 Related Issue

Closes #60

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🛠 Changes Made

• Library Integration: Added the SheetJS (xlsx) library via CDN in index.html
  to handle file generation.
• UI Enhancement: Added a new "Export to Excel" button in the filter section, styled with a distinct green theme (including dark mode support).
• Export Logic: Implemented exportToExcel() in script.js which:
  • Respects all active UI filters (Search, Month, Category) and sorting.
  • Formats data with clear headers: Date, Category, Description, and Amount (₹).
  • Automatically adds a TOTAL row at the bottom.
  • Includes automatic column width adjustments for better readability

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📷 Screenshots (if applicable)

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✅ Checklist

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