(feat) Show messages' timestamp in UI

[tobitege]

Short description of the problem this fixes or functionality that this introduces. This may be used for the CHANGELOG

Added timestamp display for each chat message.

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Give a summary of what the PR does, explaining any non-trivial design decisions

• chat-related UI components and tests extended to take timestamp into account
• aria-label for chat messages with translations for screen readers, like Message from Assistant
• tweaks in backend for consistent datetime handling

TODO?

• the date format of the user's browser or system should already be used, need confirmation from other regions
• is extra styling needed? is the frontend code meeting standards?

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Link of any specific issues this addresses

[tobitege]

How come the pre-commit doesn't run prettier on changed frontend files, but the workflow errors on them? grrr

[tobitege]

Thanks for all your expert advice, @amanape ! 🏅