Add owners management UI for published assistants

[Copilot]

Published assistants support multiple owners in the backend (owner_ids field), but the frontend lacked any interface to manage them beyond the initial creator.

Changes

New Component: OwnersStep

• Input-based UI for adding/removing owners via lhmObjectID
• Step 6 in EditAssistantDialog (published assistants only)
• Validation: prevents empty input, duplicates, and removing the last owner
• Visual "You" indicator for current user

State Management

• Added updateOwnerIds callback to useAssistantState hook
• Wired to existing owner_ids state, included in save payload

Integration

Step order for published assistants:

1. Details → 2. Tools → 3. Quick Prompts → 4. Examples → 5. Visibility → 6. Owners → 7. Advanced Settings

i18n

Complete translations for all owner management strings (de, en, bar, fr, uk)

Implementation Notes

• Uses onKeyDown for Enter key handling
• Disables delete button when ownerIds.length === 1
• No backend changes required - API already accepts owner_ids array in update payload

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