Fix playlist not clearing when navigating back (#8888)

[Elitenotavailable]

What is it?

• Bugfix (user facing)
• Feature (user facing)
• Codebase improvement (dev facing)
• Meta improvement to the project (dev facing)

Description of the changes in your PR

• Migrated "Remove watched" and "Remove duplicates" logic from LocalPlaylistFragment to a new LocalPlaylistViewModel (Kotlin).
• Used viewModelScope to ensure that database mutations survive fragment destruction when a user navigates back quickly after confirmation.
• Refactored LocalPlaylistFragment to observe the ViewModel's workState for managing the loading spinner and error reporting.
• This addresses the architectural root cause where RxJava disposables were cleared in onDestroyView, prematurely cancelling pending database writes.

Before/After Screenshots/Screen Record

• The UI remains identical, but the background operation now successfully completes after navigating away.

Fixes the following issue(s)

• Fixes Playlist not cleared if going back quickly #8888

Relies on the following changes

• None

APK testing

The APK can be found by going to the "Checks" tab below the title. On the left pane, click on "CI", scroll down to "artifacts" and click "app" to download the zip file which contains the debug APK of this PR. You can find more info and a video demonstration on this wiki page.

Due diligence

• I read the contribution guidelines.
• The proposed changes follow the AI policy.
• I tested the changes using an emulator or a physical device.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Elitenotavailable]

I am new so how long does it usually takes to review a pull request ?

[TobiGr]

@Elitenotavailable This depends on how complicated the PR is and if there is enough free time to work on NewPipe. Was this PR submitted as part of a university or high scholl course? There is a similar PR that is migrating the playlist to view model.

@Isira-Seneviratne Can you take a look at your PR and this one? Our plan is to stop working on the refactor branch and merge directly to dev (see #13635). I think that you introduced a separate library for paging.

[Elitenotavailable]

@TobiGr Actually Yes , This can be said as a part of university project , I need to contribute to big open source projects like more than a 100 stars to apply for the open source development community of my college next week and I am also Learning :) , this is my first PR