Purchase Order Type Dashboard

This module adds a new Dashboard menu under the Purchase app. It displays each Purchase Order Type as a card with live counters, so buyers can see at a glance how many requests for quotation and purchase orders exist for each type.

• The dashboard is available in kanban, list, form and pivot views, and each card shows the number of RFQs, the number of purchase orders with nothing to bill yet, and the number of purchase orders waiting for bills.
• Clicking a counter opens the matching purchase orders, already filtered by that order type and by the relevant status.
• On the Purchase Order Type form, three stat buttons appear in the button box: one for RFQs, one for orders with nothing to bill, and one for orders waiting for bills.
• The Purchase Order Type list view also shows the same three counters, with column totals.
• A new 'Nothing to Bill' filter is added to the purchase order search view, so users can filter orders that have not been invoiced yet.

The counters are computed automatically from the purchase orders linked to each type: RFQs count draft, sent and to-approve orders; 'Nothing to Bill' counts confirmed or done orders whose invoice status is 'no'; 'Waiting Bills' counts orders whose invoice status is 'to invoice'. The module extends purchase.order.type and reads purchase.order data; it does not change the purchase order workflow itself.