fix(virtual-bg): stop effect when not in use

[Antreesy]

☑️ Resolves

• in VideoStreamBackgroundEffect, stopEffect() is not properly called on disabling video stream, which leads to:
  • TimerWorker still running
  • _renderMask still called
  • mask still calculated on the black frame (taking 50% of time/resources compared to enabled video)
• During call:
  • _stopEffect() on a node processor stopping bg effect regardless of input stream state
• While checking devices:
  • effect lives as long as dialog is opened (no parallel inference during call)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

Running the same pattern:

1. Camera off, blur off
2. Camera on, blur off
3. Camera on, blur on
4. Camera off, blur on (fix)

🏚️ Before	🏡 After

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Integrations with Files sidebar and other apps
  • Not risky to browser differences / client
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

[Antreesy]

/backport to stable34

[Antreesy]

/backport to stable33

[Antreesy]

/backport to stable32