Supplier statement reconciliation and document merging

Briefcase now automatically reconciles supplier statements with transactions through a five-step process:

1. Upload documents via email, WhatsApp, or web
2. Briefcase identifies and processes supplier statements automatically
3. Statements are matched against existing transactions (partial matches require confirmation)
4. New transactions auto-reconcile against unreconciled statements
5. Manual reconciliation available when no matches are found

Users can combine multiple documents belonging to the same transaction by:

1. Selecting transactions in Costs inbox
2. Clicking "Merge"
3. Choosing primary transaction – attachments from other transactions transfer here
4. Secondary transactions are archived with merge reference

Users can now remove attachments from any transaction using the trash button.

Prepayment journals now post with 'No VAT' tax rate instead of inheriting the original invoice's VAT rate, preventing journals from appearing on Box 4 reporting on VAT returns.