text: Title: JS slugify (with unicode enabled) leaves invalid characters in the slug
  Body: ### Issue Summary
  
  The JS `slugify` function (used when cleaning a manually-entered slug) fails to strip out certain spacer/combining characters that are disallowed by Django slug validation.
  
  ### Steps to Reproduce
  
  1. On a site with `WAGTAIL_ALLOW_UNICODE_SLUGS = True`, create a new page
  2. Paste the text "উইকিপিডিয়ায় স্বাগতম!" into the slug field
  3. Defocus the field; it will be rewritten to "উইকিপিডিয়ায়-স্বাগতম"
  4. Attempt to save the page; this returns the error "Enter a valid “slug” consisting of Unicode letters, numbers, underscores, or hyphens."
  
  The expected behaviour is that the browser-side slug cleanup step should result in a slug that is accepted by the Django validation. However, the output contains the character [U+09BF](https://www.fileformat.info/info/unicode/char/09bf/index.htm) which is disallowed, being a spacing combining character.
  
  Note that the bug only arises for slugs entered manually into the slug field; pasting "উইকিপিডিয়ায় স্বাগতম!" into the _title_ field results in the slug "উইকপডযয-সবগতম" which is valid. This is because the title-to-slug conversion uses a more complex function which also performs transliteration when unicode slugs are disabled (which wouldn't be appropriate for manually-entered slugs); however, the simplified function used for cleaning manually-entered slugs is _overly_ simple:
  
  https://github.com/wagtail/wagtail/blob/84b3bf70349171e44f283085f1d89ccd9deed40e/client/src/utils/slugify.ts#L10-L13
  
  since it's only stripping out a designated short list of punctuation characters, where it would be better to strip out everything that is _not_ classed as a letter/number or explicitly allowed punctuation (just hyphen and underscore I think). Would be useful to compare this against the behaviour of the more advanced title-to-slug conversion, and Django's `django.utils.text.slugify`.
  
  -   I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: yes
  
  ### Technical details
  
  -   Python version: 3.12.4.
  -   Django version: 5.1.1
  -   Wagtail version: 6.3a0
  -   Browser version: Chrome 128
  
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