Fix notebook save empty dataset

[Creylay]

Summary

Saving a dataset from a notebook after removing all columns via converters caused a crash in the backend job (KeyError, ZeroDivisionError) because compute_metadata iterated over stale column type metadata that no longer matched the actual dataframe. The root cause was that remove_columns in DashAIDataset removed columns from the data but never updated self.types, so the Arrow metadata on disk stayed out of sync. Additionally, the frontend offered no feedback — the save button remained active even with an empty result.

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Type of Change

• Backend change
• Frontend change
• CI / Workflow change
• Build / Packaging change
• Bug fix
• Documentation

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Changes (by file)

• DashAI/back/dataloaders/classes/dashai_dataset.py: fixed remove_columns to also remove deleted columns from self.types, keeping Arrow metadata in sync with the actual data; added early-return guards in _compute_numeric_metadata, _compute_categorical_metadata, _compute_correlations, and _compute_quality_metadata to handle dataframes whose columns no longer match the stored type metadata.
• DashAI/front/src/components/notebooks/notebook/DatasetPreviewNotebook.jsx: added totalRows state that re-fetches the row count after each converter change; passes hasNoColumns to SaveDatasetModal when the result has 0 rows.
• DashAI/front/src/components/notebooks/datasetCreation/SaveDatasetModal.jsx: added hasNoColumns prop that disables the submit button and shows an error message when the dataset is empty.
• DashAI/front/src/utils/i18n/locales/*/datasets.json: added error.cannotSaveEmptyDataset translation key in all five languages.

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Testing

• Apply a ColumnRemover converter that removes all columns from a notebook, then open "Save as New Dataset". The submit button should be disabled with an error message.
• Apply converters that only remove some columns, save should proceed normally.