Dashboard widget: warranties expiring in the next 60 days

The warrantyExpiringTools() rollup helper from the previous iter now has a UI surface. ToolWarrantyExpiringCard renders the soonest-to-expire tools (max 5 inline + a `see all` link) on /dashboard, so fleet managers and accountants spot warranty risk at the same glance as low-stock and PM-overdue. Card returns null when nothing's in the 60-day window — a clean dashboard means no work. 4 new tests for the formatDaysUntilExpiry helper cover singular/plural/zero/fractional/negative inputs.