feat: dataset import dedup round-trip and prompt import/export

[LIYUE1918]

Summary

Dataset Import Improvements

• Import dedup: Skip/create-update existing datasets by \id, \questionId, or question text to avoid duplicates on re-import
• Export round-trip: Added _easyDatasetId\ / _easyDatasetQuestionId\ to custom CSV/JSON exports, enabling lossless re-import
• Field mapping: Auto-skip metadata fields (_easyDatasetId, _easyDatasetQuestionId) during import field auto-detection
• Import progress: Show separate "created" / "updated" counts in the import progress UI

Prompt Import/Export

• Export all prompts: New API \GET /api/projects/:id/prompts/export\ and UI button to download all prompt templates + custom overrides as JSON
• Import prompts: New API \POST /api/projects/:id/prompts/import\ with preview dialog showing what will be imported (merge or replace mode)
• New \getAllCustomPrompts\ / \clearAllCustomPrompts\ DB functions
• New \PromptSelector\ component for selecting prompts across categories
• i18n strings for all 5 locales (en, it, pt-BR, tr, zh-CN)

Fixes

• ActionBar icon swap: Fixed reversed icons (Import now shows download icon, Export shows upload icon)

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[LIYUE1918]