Implement Expenses Page

• Design and implement the frontend UI for the Expenses Page.
• Create API endpoints for fetching, adding, updating, and deleting expense records.
• Implement a table listing all expense transactions (date, category, amount, notes).
• Add filters to sort expenses by category and date.
• Ensure that if there are more than N expenses, pagination is available.
• Display a pie chart or bar graph to visualise spending distribution by category.
• Allow users to mark recurring expenses (e.g., rent, subscriptions).
• Ensure authentication is required for all expense-related actions.

Closes #11 (closed)