Fix page publication fields not clearing when unchecking published checkbox

[antwertinger]

What is this pull request for?

When unchecking the "published" checkbox on a page that already has a public_on date set, the page remained published after saving. The root cause was that the code set input.value = "" directly on the raw element, which flatpickr ignores — it uses its own internal state for form submission.

Fix: query the web component instead of the raw input and call flatpickr.clear() to correctly reset both the DOM value and flatpickr's internal state.

Notable changes (remove if none)

• page_publication_fields.js now selects alchemy-datepicker elements via :has() selector instead of the inner directly
• Uses flatpickr.clear() instead of input.value = "" to properly clear the datepicker state on uncheck

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have added tests to cover this change

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (a5e74a4) to head (43ce4ac).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3737   +/-   ##
=======================================
  Coverage   97.47%   97.47%           
=======================================
  Files         314      314           
  Lines        8303     8303           
=======================================
  Hits         8093     8093           
  Misses        210      210           

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