UI: multi-select + batch operations

[deOliveira-R]

Zotero users frequently need to act on several items at once: add to a collection, tag, delete, export BibTeX.

Desired batch actions (v1 set)

• Add selected items to an existing collection.
• Create a new collection from selection.
• Add/remove a tag across selection.
• Export BibTeX (concatenated `.bib`) for selection.

UI sketch

• Checkbox appears on item rows on hover (or always, toggle via a setting).
• Shift-click range selection; Cmd/Ctrl-click toggle.
• Bottom bar appears when ≥ 1 item is selected: `[N selected] [Add to Collection ▾] [Tag ▾] [Export BibTeX] [Cancel]`.
• Routes: `POST /items/batch` with an `action` and `ids[]` body.

Deletion

Skip deletion in v1 — grimoire's MCP / CLI doesn't expose it either, and accidental deletes are a bigger cost than convenience gain. Revisit with an "archive" soft-delete model later.

Acceptance

• Multi-select UI works with keyboard (see UI: keyboard navigation (j/k/enter, /, esc, g/G) #7) and mouse.
• Bulk BibTeX export returns a single `.bib` file.
• Bulk add-to-collection + add-tag operations are atomic (single SQL transaction).
• No deletion in v1.