fix(cart): store whole-line basket prices and send them as end_amount

[mekedron]

Summary

A basket built with wolt cart add --count 2 showed three different totals.
The Wolt app counted each such line once, wolt cart showed its own number,
and wolt checkout counted lines added in the Wolt app twice. For example,
a 37.80 basket previewed at 75.60.

Wolt stores a basket line's price as the total for the whole line. It reads
the checkout end_amount as that same total. The CLI stored the price of one
unit and multiplied it by the count again for checkout. Now the CLI stores
and sends prices the same way the Wolt app does. All three totals match.

Baskets built by older CLI versions still hold one-unit prices. Wolt shows
them too low. Rebuild such a basket after upgrading. If a line's price cannot
be split evenly by its count, the CLI refuses it and asks you to add the item
again.

Fixes #59 — #59

What wolt.com sends

Captured from wolt.com network traffic at Sultan Shish Kebab, Espoo:

Line	Basket price	Plan base_price	Plan end_amount
2 x Coca-Cola at 3.50	700	350	700
2 x Kebab with fries at 14.40 + Yogurt 1.00	3080	1440	3080

Wolt's "Item subtotal" for that plan was 37.80 = 7.00 + 30.80. Option values
keep their per-unit price and count (100, count 1).

This settles the open question in the issue. end_amount is not a per-unit
amount. Wolt only appeared to multiply by count because the CLI sent count x
price while the basket held a unit price. With whole-line prices on both
sides, the totals agree.

Changes

• cart add (CLI and MCP) saves count x (unit price + paid option values).
• BuildBasketUpsertItem rescales a count-based line's price whenever a merge
  or a partial removal changes its count. Weighted lines keep the price their
  weight builders compute.
• cart show sums line prices for the subtotal. It reports price as one
  unit (line price / count) and line_total as the line price.
• checkout sends the line price as end_amount. It sends one unit without
  options as base_price. Weighted lines are unchanged.
• The live smoke adds two units of the fixture item. It checks the line math,
  compares Wolt's own basket total with the CLI subtotal, and compares the
  checkout item subtotal with both.
• Docs: the output contract and the cart add text describe the line price
  as the whole-line total.

Testing

• Unit and e2e tests cover rescaling on merge and partial remove, rejecting
  an indivisible price, weighted lines, cart show totals, the checkout plan
  and MCP add/remove.
• go build, the versioned bin/wolt build and version check,
  golangci-lint run, go test ./... and go test -race ./... all pass.
• Live check on the real account, at the basket created on wolt.com:
  • the current main previewed an item subtotal of 75.60;
  • this branch previewed 37.80, the same as wolt.com.
• After adding, merging and partially removing lines through this branch,
  three totals matched at every step: Wolt's basket total, the CLI subtotal
  and the checkout item subtotal. The last step was 69.10 for 1 x 15.40,
  3 x 3.50 and 3 x 14.40.
• WOLT_SMOKE_CART=1 ./scripts/live-smoke.sh passed 29 of 29 steps. The new
  two-unit checks passed. The weighted leg was skipped because no
  sell-by-weight item was orderable at that hour.
• Not checked live: a weighted line together with a count-based line in one
  checkout. The weighted code path is unchanged.