Founding offer: show days left until seats are actually taken

[danielehrhardt]

The pricing pill showed "100 of 100 left", which reads as "nobody bought this".

• Until 10 founding seats are really taken (FOUNDING.showCountFrom), the pill shows the deadline: "24 days left · closes October 31", with the bar showing how much of the offer window has passed. Days are rounded down, so the timer never overstates.
• From 10 real seats on it switches to the real count ("N of 100 seats left"), and to "Founding seats are gone" when the offer closes.
• /api/offer only includes left once the count is shown, so the number isn't exposed below the threshold.
• No numbers are invented: seat logic, checkout and terms are unchanged.

Checked: astro check 0 errors, production build, local run against a migrated local D1.