refactor: static content assets directory

[Schwehn42]

Description

Resolves #4736

Changelog

• Move timer_finished.mp3 and hotkeys.pdf from public to public/assets
• Adjust path refs

[github-actions]

The deployment to the dev cluster was successful. You can find the deployment here: https://4775.development.scrumlr.fra.ics.inovex.io
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• ghcr.io/inovex/scrumlr.io/scrumlr-frontend:sha-2dd9936

• ghcr.io/inovex/scrumlr.io/scrumlr-server:sha-2dd9936

[octomind-dev]

🐙 Octomind

Test Report: 14/14 successful.

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commit sha: 2dd9936