Library: multi-select datasets with bulk pipeline, training, and delete actions

[mattdawkins]

• The desktop Library (Recents) table gains a selection checkbox column on the right, replacing the per-row delete button; the column header is a select-all (indeterminate when partial, follows the active search filter)
• When anything is selected, Run Pipeline, Run Training, and Delete (N) buttons appear to the left of the search box (which keeps its position); delete shows one confirmation for all selected datasets (source media and originally imported annotation files are untouched)
• Run Pipeline / Run Training navigate to their pages with the checked datasets pre-staged via a datasetIds query parameter
• Bulk pipeline runs now launch only on staged datasets compatible with the selected pipeline

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