Show overdue invoice alerts on the dashboard

[grmbyrn]

Problem

There is no at-a-glance view of outstanding or overdue invoices.
Users have to navigate to the invoices page to discover this.

Proposed Solution

Add a summary widget to the dashboard showing unpaid invoice totals
and a count of overdue invoices.

Acceptance Criteria

• Dashboard shows total value of unpaid invoices
• Dashboard shows count of overdue invoices with a warning colour
• Clicking the widget navigates to the invoices page filtered by that status
• Widget is only shown when there are relevant invoices (no empty state clutter)

Labels

enhancement