fix(orders): let customers change pickup slot on PRINTED orders (frontend)

[choden-dev]

Problem

PR #100 updated the POST /api/shop/:orderId/select-timeslot endpoint to accept PRINTED orders, so the backend now supports customers changing their pickup slot after an admin has printed the order.

However, the customer-facing frontend never surfaced a way to trigger this, so in practice only admins could change the timeslot of a printed order. Two frontend touchpoints gated on PAID/AWAITING_PICKUP only:

1. pages/my-orders.tsx — the Change Pickup button in renderOrderAction was only rendered for PAID and AWAITING_PICKUP orders. A customer with a PRINTED order saw no action button.
2. components/ordercontainer/OrderContainer.tsx — navigating via ?pickupFor=<id> only forced the PICKUP step for PAID/AWAITING_PICKUP. For a PRINTED order, statusToStep() returned COMPLETE, dropping the user on the confirmation screen instead of the timeslot picker.

Fix

• my-orders.tsx: show the Change Pickup button for PRINTED orders too.
• OrderContainer.tsx: force the PICKUP step for PRINTED orders when arriving via ?pickupFor=.

The PICKUP step in OrderSteps.tsx already renders TimeslotSelector unconditionally (and OrderSteps already allows staying on the PICKUP step for PRINTED), so no further changes were needed once the step is reached. The COMPLETE step's existing Change Pickup button also now works end-to-end for printed orders.

Testing

• biome check passes on both changed files.
• tsc --noEmit passes with no errors.
• Pre-commit and pre-push hooks passed.