fix(core): CATALYST-1579 Replace checkout redirect server action with /checkout route

[jordanarldt]

What/Why?

We found that the redirectToCheckout server action was a bit unnecessary, as it was simply checking if the cart exists before redirecting to /checkout, which would also then check the cart and redirect to the checkout route.

By removing the server action and updating the checkout button to simply GET /checkout, we're skipping an unnecessary step which should improve performance by a bit.

Additionally, I updated the translations to return better user-facing errors instead of mentioning a cart cookie, and also updated the /checkout route to return toast messages on errors.

Testing

Manual testing locally

Before change - proceed to checkout takes several seconds

before_change.mov

After change - proceed to checkout takes about 1 second

after_change.mov

Migration

1. Remove core/app/[locale]/(default)/cart/_actions/redirect-to-checkout.ts
2. Update checkoutAction in core/app/[locale]/(default)/cart/page.tsx to "/checkout"
3. Copy changes to core/app/[locale]/(default)/checkout/route.ts
4. Update core/lib/server-toast.ts and set the cookie maxAge to 1 - this ensures any toast errors live through the redirect back to the /cart page
5. Copy changes in core/vibes/soul/sections/cart/client.tsx
6. Update en.json with the updated translation values (optional)

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