feat: add loading indicator to history and templates

[Schwehn42]

Description

Users with many templates or especially boards don't get a visual indicator while they are being fetched (not having pagination doesn't help the cause).
In fact, on the history page, the user might falsely assume they have no boards at all since the empty boards text is being displayed.

This PR not only adds a simple loading indicator to both Templates and History, but also uses a small hook to prevent flickering when the delay is marginal.

Changelog

• Templates, History: add loading indicator while thunk fetch hasn't completed yet
• add new hook useDelayedLoading, which essentially debounces the loading value so only after a certain loading time (200ms as of now) the indicator is actually rendered.

Checklist

• I have performed a self-review of my own code
• I have written and understand every part of this contribution myself - if AI tools were used, I have thoroughly reviewed and verified all changes
• I have commented my code, particularly in hard-to-understand areas
• The light- and dark-theme are both supported and tested
• The design was implemented and is responsive for all devices and screen sizes
• The application was tested in the most commonly used browsers (e.g. Chrome, Firefox, Safari)

[github-actions]

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[Planlos5000]

LGTM