[translator] Configurable single/multi target-language selection

[denis-urban]

Summary

The bulk dashboard and per-document panel translate into one target language per run. Add a plugin option to allow selecting multiple target languages in a single run, configurable per install (default stays single for backward compatibility).

Current behavior

• Both forms render the target as a single-value select (FormSelectLocale bound to FORM_FIELDS.TARGET_LNG).
• Form schema: target_lng: z.string().nonempty().
• The enqueue endpoint (EnqueueInputSchema) accepts a single target_lng; the handler builds one task per selected document.

Proposed change

Add a config option, e.g.:

translatorPlugin({
  // ...
  targetSelection: 'single' | 'multi', // default: 'single'
})

• single → unchanged.
• multi → the target field becomes a multi-select; enqueue accepts target_lng: string | string[] and fans out one task per (document × target language). The handler already loops over selected documents — add an inner loop over target languages.

Acceptance criteria

• Default (single) behavior is unchanged.
• multi lets an editor pick N target languages; a run queues all of them.
• Progress/status (get-collection-status) reflects every queued target.
• The enqueue endpoint accepts both string and string[] for target_lng (back-compat).

Notes

• Source-language selection stays single.
• Existing-translation / in-progress indicators per target language are a separate enhancement.