fix(billing): send the plan name from route params under Next 16

[vernu]

/checkout/pro and /checkout/scale failed with "Plan cannot be purchased" for every user since the Next 16 upgrade (ea1b00c).

The checkout page still typed params as a plain object and read params.planName synchronously. Next 16 removed the sync compat shim, so the value was undefined, axios dropped the key, and the API ran findOne({ name: undefined }). Mongoose 9 returns null for that where older versions stripped the key, so the guard threw a message about a misconfigured plan for a request that never named one. Plan data and Polar product ids were correct throughout.

• unwrap params with React's use() and drop the as string cast that suppressed the type error
• split the conflated guard into PLAN_NAME_REQUIRED, PLAN_NOT_FOUND and the existing unpurchasable case, each logged server side
• stop retrying a rejected checkout twice: a 400 is not transient
• mark the Polar product id indexes sparse, they have been failing to build with E11000 on every boot because most plans leave them unset

Checkout now defaults to yearly. A URL that names an interval still redirects straight through, so the marketing funnel is unchanged; the in-app CTAs that name none get an interval chooser with yearly preselected instead of a silent monthly default.

Also redesigns the checkout states, which were the only place in the dashboard hardcoding gray-100/white and so rendering a light slab in dark mode, and the plan picker, which painted the accent on every tier.

Tests: a checkout e2e spec asserting the request body carries planName, which is what would have caught this, plus API cases pinning the three guard failures apart.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
textbee	Ready	Preview, Comment	Jul 20, 2026 7:11pm