Improved the expense tracker website

[basundhara-070]

Summary

• Added two graphs - bar graph to show monthly expenses and a pie chart to show category-wise expenses.
• Added filter by category feature.
• Improved the overall UI.
• Made the data persist to local storage.

Describe your changes clearly.

Checklist

• Linked issue (if any)
• Ran locally without console errors
• Focused scope (kept PR small)
• Updated docs or TODO comments if needed

Screenshots

Before changes:

After changes with working:
screen-capture (3).webm

Closes #67

[vercel]

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