feat: Add Export/Import functionality for Mood & Journal History

[ATOMworkplace]

PR Description
Description This PR introduces data persistence features, allowing users to export their session history (Moods and Journal entries) to CSV or PDF formats and restore them later via CSV import. This resolves the need for users to save their mental health tracking data.

Changes Made

• New Feature: Added a "Data & Settings" page in the sidebar.
• Export: implemented CSV download for Mood/Journal history using pandas and PDF report generation using fpdf.
• Import: Added functionality to upload CSV files and restore history into the current session.
• State Management: Updated st.session_state to track mood_history and journal_history lists instead of just displaying immediate results.
• Dependencies: Added pandas and fpdf to requirements.txt.

Related Issue Closes #5

How to Test

• Run the app: streamlit run app.py.
• Go to "Mood" or "Journal" and add a few entries.
• Navigate to the "Data & Settings" page.
• Try downloading the CSV and PDF.
• Refresh the page (to clear session) and use the "Import" tab to upload the previously downloaded CSV. Verify the data is restored.

Preview