Add two-sided station article transfer picker

[osswald]

Summary

• Replace the flat station article multi-select with a two-sided transfer picker: searchable category tree on the left, grouped selected list with filter on the right; click moves articles between sides.
• Extend minimal article catalog responses with article_category_id / article_category_name so category labels render correctly.
• Prune layout cell article IDs that are no longer on any station during config autosave to avoid 422 validation errors when rapidly transferring articles.

Test plan

• Open event config → Stationen and confirm available/selected panels with category labels
• Add several articles left→right and remove several right→left in quick succession; autosave succeeds without 422
• Filter both sides and confirm matching results / empty states
• Confirm layout button articles that are removed from all stations are dropped on save (no stale subset violations)
• cd cloud/frontend && npm test && npm run typecheck
• ./scripts/lint.sh

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