Bulk supplier update for documents

Why

The AI can incorrectly assign documents to similar but wrong suppliers (e.g., "Travis Perkins Hire" instead of "Travis Perkins"). Whilst improved matching logic will reduce this (COR-1479), when it does occur, users must manually update each document individually. This is inefficient when multiple documents need the same supplier correction.

What

Enable bulk supplier updates in the documents view:

• Allow users to select multiple documents
• Provide option to change supplier for all selected documents at once
• Update supplier field across all selected documents in a single action

How

• Add "Change supplier" bulk action to documents page when multiple documents selected
• When clicked, open supplier selection dropdown/modal
• Upon confirming new supplier, update all selected documents
• Show confirmation message with count of documents updated
• Ensure action is available in the bulk actions toolbar alongside existing Delete/Approve actions

Dependencies

• COR-1479 - Improved supplier matching should reduce frequency of this issue

Acceptance Criteria

• Users can select multiple documents in documents view
• "Change supplier" option appears in bulk actions when documents are selected
• Clicking action opens supplier selection interface
• All selected documents update to new supplier when confirmed
• Action works regardless of current supplier assignments
• Confirmation message shows number of documents updated
• Works consistently with existing bulk actions (Delete, Approve)