Fixes #27

[TriptiMirani]

This pull request introduces functionality for exporting chat history as a PDF file, allowing users to print, open, and share their conversation records on clicking the download button on the top-right corner of chat_page. The key features include:

• Chat Retrieval & Decryption: Fetches chat history from the SQLite database and decrypts messages using RSA for better readability.
• PDF Generation: Formats chat messages into a structured PDF layout, including timestamps and message types (text, file, voice).
• Print Support: Enables direct printing of the generated PDF using the printing package.
• Open PDF: Opens the generated PDF using the default file viewer.
• Share PDF: Allows users to share chat history via external apps (e.g., WhatsApp, Gmail) using share_plus.
• Error Handling & UI Integration: Displays appropriate messages for failures and integrates buttons in ChatExportScreen for user interaction.

This enhancement improves accessibility and usability, enabling users to securely store and share their chat history. 🚀

Here is an example of conversation between Jake Peralta and Amy Santiago.

Screenshot from Jake's end:

PDF Generated on Jake's end:
chat_history_1740073806094.pdf

Screenshot from Amy's end:

PDF generated on Amy's end:
chat_history_1740074296098.pdf

Note:

1. We can't see emojis in the pdf as the PDF library doesn't handle emoji characters well.
2. We can't see the image being sent through the chat, which is a bug that needs to be addressed.