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

[Antreesy]

☑️ Resolves

• 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

[backportbot]

The backport to stable32 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable32
git pull origin stable32

# Create the new backport branch
git checkout -b backport/18205/stable32

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 102ef57e

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/18205/stable32

Error: Failed to check for changes with origin/stable32: No changes found in backport branch

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.