Build cart route (qty +/-, remove, clear, total, empty state)

## Goal
Build the `/cart` page listing items with quantity controls, remove button, clear-cart button, running total, and a friendly empty state.

## Acceptance criteria
- [ ] Empty state shows a CTA back to the menu
- [ ] Each row shows image, name, qty +/-, remove button, line total
- [ ] Decrementing quantity to 0 removes the item
- [ ] Total updates live
- [ ] Playwright test covers add, update qty, remove, and persistence across reload

Start at the `/cart` route and trace how menu items are added and persisted. Build the cart page against the listed acceptance criteria, then add a Playwright test covering add, quantity updates, removal, and persistence across reload; done means all controls, totals, empty state, and CTA work.

Written by the indexing model from the issue text.