Add multi-currency support with dynamic conversion and formatting

[swathiiie]

Currently, the Expense Tracker supports only a single default currency. Users cannot switch between currencies or track expenses in different monetary units.

This limits usability for users in different countries.

Problem

• No currency selection option
• All expenses stored in one fixed currency
• No currency formatting
• Not suitable for international users

Proposed Enhancement

Implement a multi-currency system that allows users to:

• Select preferred currency (USD, INR, EUR, etc.)
• View totals converted dynamically
• Format amounts according to currency
• Persist selected currency across sessions

Core Features

Currency Selector Dropdown
Dynamic currency formatting
Real-time conversion (static rates or API-based)
Persistent user preference (localStorage)
Automatic total recalculation when currency changes