Fixing credits, cookies, and sign in

A handful of fixes, across builds, the marketing pages and sign in:

• Builder: starting a build without enough credits created the app first and rejected the run second, leaving a dead tile on your home screen with no build behind it and no lasting error. Credits are reserved before anything is created, so a rejected build leaves nothing behind.
• Marketing pages: a second, dead copy of the cookie banner was baked into the pages themselves, so it came back on every hard reload even after you had already answered it. You are asked once now, and it stays answered.
• Sign in: getting sent to sign in from a page that needed an account now returns you to that page afterwards, rather than dropping you somewhere else.
• Accounts: confirming an account that was already verified now requires a valid code.
• Marketing header: the signup call to action no longer shows up when you are already signed in.