[FEATURE] Add CSV export functionality

[devmaster1987]

Description

Add an option that allows users to export their expense tracker data into a CSV file.

Currently, users can backup their data using JSON export, but there is no option to download transaction data in a spreadsheet-friendly format.

Current Problem

• Users cannot export transactions to CSV format
• Data cannot be easily opened in Excel or Google Sheets
• No simple reporting option available

Expected Behavior

• User should be able to export all transactions as a CSV file

• CSV file should contain important transaction details:

  • Transaction ID
  • Title
  • Amount
  • Type
  • Category
  • Date
  • Notes

• Exported file should open correctly in spreadsheet applications

Suggested Solution

• Add an "Export CSV" button in Data Management section
• Convert transaction array into CSV format using JavaScript
• Generate downloadable CSV file
• Add proper file naming convention

Example: