fix: pause video playback on screen change using VisibilityDetector

[surelyruchi]

PR Description

This PR fixes the issue where the video player continued playing in the background even after navigating away from the screen.

🛠️ How It’s Fixed

• Integrated VisibilityDetector to monitor the visibility of the video widget.
• When the widget is no longer visible (i.e. screen changes), the video playback is paused.

🎯 Why This Approach

• VisibilityDetector is a lightweight and effective way to track widget visibility without relying on screen lifecycle events.
• It avoids unnecessary resource usage and ensures a cleaner user experience.

Related Issues

• Closes Switching to a different screen does not stop the background tasks from the previous screen #848

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• Yes
• No, and this is why: it's been manually tested, and we can consider writing a test later if needed based on feedback.

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux

[surelyruchi]

Hey @ashitaprasad @animator
Let me know your reviews on this.
Just a few things I wanted to mention about this PR :

• I removed _videoController.play() from initState to avoid auto-playing if the user navigates away while the request is still in process, and the play will be handled by the Visibility Detector.
• Handled manual pause case using the existing _isPlaying variable to ensure the video doesn't auto-play again if the user paused it intentionally.
• Auto-play resumes only when visibility ≥ 50% and the user hasn’t manually paused.