Migrate cart-overlay and quote-cart styles into the design system

[stefan-burke]

Summary

• Move the cart-overlay dialog (.cart-item, .cart-item-info, .cart-item-controls, .cart-checkout, etc.), the quote-cart family, and the quote-price summary out of the legacy src/css/cart.scss into new design-system partials (src/css/design-system/_cart.scss, _quote-cart.scss).
• The new partials use design-system tokens ($space-*, $border-light, $radius-md) and mixins (button-primary, button-ghost, flex-col, flex-between, dialog-box) instead of the raw values that were sprinkled through cart.scss.
• Wrap theme-switcher, cart-icon, and cart-overlay in a <div class="design-system"> from base.html so the design-system styles also win on legacy layouts (products listings, item-reviews, page).
• The cart-item template now wraps quantity-controls in .item-quantity to match list-item-cart-controls, so a single widget rule covers both contexts.
• Legacy cart.scss is reduced to the inline list-item widgets (still needed by layouts not yet wrapped in .design-system) plus the global dialog-scroll-lock rule that has to live outside the design-system scope.

Test plan

• bun run build succeeds and emits scoped .design-system .cart-* / .quote-* rules in both bundle.css and design-system-bundle.css.
• bun test passes (3047 unit + integration tests, design-system scoping check, copy-paste detection, lint, typecheck, build).
• Open the cart on a legacy products listing page and on a design-system page; confirm the overlay header, items, totals, and Stripe button render correctly.
• Add and remove items from a Stripe-mode cart; tweak quantities via the +/- buttons.
• Step through the quote flow (/quote/, /checkout/) and verify the quote-cart, quote-checkout summary, and sticky quote-price aside still look right.

https://claude.ai/code/session_01MyGvKCdJGipPrBLSAm13V6

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