fix(dynamic): handle dynamically inserted time elements via MutationObserver (#22)

[jcdelepine]

Title: fix(dynamic): handle dynamically inserted time elements via MutationObserver (#22)

Description:

The patch f398f58 replaced time-elements.js with relative_time.js using Intl.RelativeTimeFormat for localization. However, it lost the MutationObserver that time-elements.js used to detect <time is="time-ago"> elements inserted dynamically via AJAX.

As a result, the relative time labels were never rendered for messages loaded dynamically in the mailbox view, leaving the <time> element empty and producing output like:

Yesterday, 4:05:55 PM CEST ()

This issue may be browser and timing dependent — connectedCallback fires reliably in some environments but not others, particularly under Firefox or when elements are inserted after script execution. The MutationObserver approach is robust regardless of browser or insertion timing.

Fix by replacing the one-shot DOMContentLoaded + sweep() with an init() function that runs sweep() for pre-existing elements and installs a MutationObserver to catch future insertions.

[ralflang]

Thanks — retargeted to pr/83-relative-time-observer so we can pair this with a small fix at the AJAX insertion site in js/base.js.