feat: Integrate IOS and Android File System for missing Export functionality

[tamang29]

Checklist

• I linked PR with a related issue
• I added multiple screenshots/screencasts of my UI changes

Motivation and Context

This PR completes #359
The Capacitor iOS and Android apps are missing export functionality (SVG, JSON, PDF, and PNG).

Description

This PR integrates the FileSystem API for mobile devices to provide export functionality features to the mobile apps.

Setup and running the app in capacitor

Prerequisite:
Xcode for iOS: Download iPad emulator
Android Studio for Android emulator: Download a tablet emulator

Setup (one time):

1. npm install
2. npm run build
3. npm run capacitor:add:ios or npm run capacitor:add:android (for android)
4. npm run capacitor:assets:generate:ios or npm run capacitor:assets:generate:android (for android)
5. npm run capacitor:sync

Run the app (macOS):
npm run capacitor:open:ios (the app will open in Xcode)

Run the app (Android):
npm run capacitor:open:android (the app will open in Android Studio)

Steps for Testing

1. Run the app on Xcode for iOS or Android Studio for Android.
2. Create a diagram
3. Test export functionality:
   Export as SVG
   Export as PNG
   Export as PDF
   Export as JSON
4. Verify exports work correctly on both iOS and Android

Screenshots