Add embeddable board web-component

[hefftich]

Resolves:
nextcloud/circles#2948

Relates:
nextcloud/circles#3055

Summary

This pull request gives us the opportunity to use the deckboard in the Teams app via a web-component.

Expose nextcloud-deck-board through an opt-in loading event so other Nextcloud apps can reuse the existing board and card detail components. Isolate embedded stores, routing and sessions without changing the host URL or global session headers. Preserve active cards when opening archived card details and keep undo actions bound to their board instance.

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI (GPT-6.1 Sol)

[github-actions]

🐢 Performance warning.
It looks like the query count of the integration tests increased with this PR.
Database query count is now 202741 was 198420 (+2.17%)
Please check your code again. If you added a new test this can be expected and the base value in tests/integration/base-query-count.txt can be increased.

[github-actions]

🐢 Performance warning.
It looks like the query count of the integration tests increased with this PR.
Database query count is now 202791 was 198420 (+2.2%)
Please check your code again. If you added a new test this can be expected and the base value in tests/integration/base-query-count.txt can be increased.

[github-actions]

🐢 Performance warning.
It looks like the query count of the integration tests increased with this PR.
Database query count is now 202759 was 198420 (+2.18%)
Please check your code again. If you added a new test this can be expected and the base value in tests/integration/base-query-count.txt can be increased.