Migrate LanguageFilter to KMultiSelect

[Prashant-thakur77]

Summary

Migrates LanguageFilter from Vuetify's VAutocomplete to KDS KMultiSelect, as part of moving Studio off Vuetify.

KMultiSelect renders its own chips and options, so the hand-rolled #selection and #item slot templates, the StudioChip import, the search helper, the scroll-reset method, and the Vuetify-specific styles all go away.

Notable points:

• Search parity is preserved. The old field passed a function as item-text returning
  name + related_names + id, then overrode both slots to display only name the
  concatenation existed purely for filtering. That's now expressed directly as
  itemText="name" for display plus :searchKeys="['related_names', 'id']" for matching,
  so searching by native name, English name, or language code all still work.
• StudioChip is no longer needed here because KMultiSelect renders chips itself using
  KChip, which is the same component upstreamed into KDS (identical markup and styles).
  No spec file existed for this component and none is added; behavior was verified as
  described below.

References

• KDS component PR: Implement the KMultiSelect wrapper and Dropdown components for KMultiSelect kolibri-design-system#1296
• Requires KDS 5.9.0, already on unstable via Upgrade KDS to 5.9.0 #6090.

Reviewer guidance

Check for no regrressions then before and proper refractoring.

Screencast.From.2026-08-17.02-43-29.mp4

AI usage

I used Claude to write this description and made the changes as needed.

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[Prashant-thakur77]

@LianaHarris360 I’ve created the PR for the LanguageFilter with KMultiSelect. :)