feat(billing): let the client request a plan, seat count and interval

[kipavy]

Client half of VoltiusApp/server#20.

openBillingCheckout sent only { plan }, so the server defaulted every purchase to monthly at three seats:

• the yearly variants were unreachable from the app, even though the pricing page leads with annual prices
• a ten-person team could not buy ten seats from the app

BillingPlan now includes business, and the function accepts optional seats and interval, omitting each when unset so every existing caller keeps its current behaviour.

No UI passes these yet — the seat and interval picker lives in the web portal, which already sends both. This is the plumbing that would let the app offer one; building that picker is a separate piece of work with real design choices.

Testing

Five new tests, the first coverage this module has had:

sends only the plan when nothing else is specified
forwards a yearly interval so the annual variant is reachable
forwards a seat count for per-seat plans
accepts the business plan
opens the checkout url the server returns

tsc --noEmit exit 0.

Order of deployment

This is inert without the server change. interval: "yearly" reaches a server that still ignores it until VoltiusApp/server#20 ships, and plan: "business" returns 400 until then and until a Business variant exists.