Multi-select and bulk-delete files in the sidebar

The file sidebar already supports building a multi-selection (Ctrl/Cmd-click and Shift-click range select, with a hover preview of the range while Shift is held). What it lacks is a way to act on that selection — specifically to delete several selected files at once without removing them one by one.

Goal: once two or more files are selected in the sidebar, offer a bulk Delete selected action (toolbar button / context menu / keyboard) that removes all selected files in one confirmed operation.

Scope / notes:

• Reuse the existing sidebar multi-selection (Ctrl/Cmd-click, Shift-click range) — no new selection model needed.
• Single confirmation dialog summarising how many files will be deleted.
• Clear the selection and refresh the list afterwards; handle partial failures gracefully.
• Respect the same permissions/guards as single-file delete.

Natural pairing with the email-list multi-select work — keeps bulk actions consistent across the list and the sidebar.