Create shipments from orders on the create page (#328)

[Connor-Finn]

Part 2 of #328.

What changed

• Entry points: "Create shipment" on an order, and "Ship together" on the orders list, open the create page with the orders attached (?orderIds=; the older ?fromOrderId= still works). "Ship together" is only enabled for two or more available orders that can share a shipment (one customer, one origin, LTL), with the reason shown otherwise.
• Orders card on the create page, with an "Add order" search limited to orders that can join.
• Editable autofill (frontend/src/lib/shipmentFromOrders.ts): orders fill empty fields, widen the dates, add each drop as a stop and turn on temperature/hazmat handling; nothing is locked any more. Saving is refused while the form no longer fits the orders, with the reasons listed.
• Lane matching: a lane from the orders' origin to their last drop that supports their service level is preselected (new GET /api/v1/lanes/between), preferring one whose stops cover the other drops. Drops it doesn't cover become extra stops, with a warning. Otherwise the shipment gets a custom route.
• The Waypoints card is only shown for a custom route; with a lane, its stops are listed in the Lane card.
• Saving attaches every order through add-orders, which enforces the same rules server-side.

Testing

• Backend suite, frontend tests, both type-checks and both lints pass.
• In the browser against a seeded database:
  • two Nordic LTL orders filled in customer, mode, the Green Bay → Denver lane with the extra Rochester stop and warning, the lane carrier and widened dates
  • switching the mode to FTL listed the conflict and blocked saving
  • saving created the shipment with both orders on the right stops
  • an FTL order on Houston → Atlanta preselected that lane and its carrier
  • "Ship together" enabled and disabled correctly for valid, single, FTL-mixed and multi-customer selections
  • "Add order" offered only compatible orders

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