feat(admin): page count & colour badges + React Query migration for orders-by-timeslot

[choden-dev]

Summary

Two related improvements to the Orders by Timeslot admin view (components/admin/OrdersByTimeslotView.tsx):

1. At-a-glance file details — show each file's page count and colour preference so the admin no longer needs to click into each order.
2. React Query migration — migrate the component off raw useEffect + fetch to @tanstack/react-query, per the roadmap in AGENTS.md.

Changes

Page count & colour badges

• Each file row in the Files column now shows two responsive badges next to the filename/copies:
  • 📄 Page count (e.g. 📄 12p) — pages per copy
  • 🎨 Colour / ⚫ B&W — colour preference, colour-coded (orange for colour, grey for B&W)
• Row uses flex-wrap so badges and the existing Open button reflow cleanly on narrow widths (responsive).
• Added pageCount to the orders-by-timeslot API response (app/api/admin/orders-by-timeslot/route.ts) so the component can render it.

React Query migration

• Replaced useState/useEffect/fetchData with useQuery keyed ["orders-by-timeslot", filter], using placeholderData: (prev) => prev.
• Converted the Mark Printed and Mark Picked Up status updates to useMutation that invalidate the ["orders-by-timeslot"] query on success.
• Extracted module-level fetchOrdersByTimeslot and updateOrderStatus helpers.
• Refresh button now calls refetch().
• Follows the pattern established in components/pickup/TimeslotSelector.tsx.

Notes

• Colour labels follow the existing project convention (COLOR → "Colour", otherwise "B&W").
• pageCount is rendered conditionally, so older data without it degrades gracefully.

Verification

• tsc --noEmit passes with no errors.
• biome check passes on all changed files.