feat: add diagnostics screen, settings link, and updated docs (#228)

[Justprotocol]

Description

This PR adds a dedicated Diagnostics Screen and links it inside the Settings tab under the About section. It allows users to view system details and export non-sensitive diagnostic logs for support and debugging purposes.

Summary of Changes

• New Feature: Added app/diagnostics.tsx to display app version, network status, node connection, and redacted log history.
• Navigation: Updated app/(tabs)/settings.tsx to add a "Diagnostics" navigation button under the About section.
• Export Functionality: Integrated native log sharing/export functionality.
• Documentation: Updated docs/diagnostics.md with guidelines on redacted data formats and log collection logic.
• Security: Verified that all private keys, seed phrases, and personal financial data are strictly filtered out before rendering/exporting.

Screenshots / Demo

(Attach screenshots of the Settings screen entry point and the new Diagnostics screen if applicable)

How to Test

1. Pull the branch: git checkout feature/diagnostics-screen-228
2. Run the app: npx expo start
3. Navigate to Settings -> About -> Diagnostics.
4. Verify system details are rendered correctly.
5. Tap Export Diagnostics Log to ensure the native share modal opens properly.

Checklist

• Code follows the style guidelines of this project.
• Tested locally on iOS/Android simulator or physical device.
• Documentation updated in docs/diagnostics.md.
• No sensitive information is exposed or logged.

Closes #228