#3949 /o/<slug>/billing sends a signed-out visitor to the collective page, or a 404 when private, instead of through login and back like /billing

Problem

/billing sends a signed-out visitor to /login?returnUrl=… and back, keeping the query string (routes/billing.tsx, pinned in tests/routes/billing_page_test.ts). /o/<slug>/billing does not:

• a public collective redirects the visitor to /o/<slug> (canViewConfig is false with no session), so after signing in they are not on the billing page
• a private collective returns 404, because loadCollectivePageContext returns null when the viewer cannot see it

The upcoming swamp subscription start --collective <slug> (swamp CLI) opens <serverUrl>/o/<slug>/billing, and prints that URL when no browser can be launched. Opening the printed URL in a browser that is not signed in, the usual case over SSH or on a headless box, lands on the wrong page or a 404.

Change

In routes/o/[slug]/billing.tsx, check the session first, before loading the collective page context. With no session user, answer 302 to /login?returnUrl=${encodeURIComponent(/o/${slug}/billing${ctx.url.search})}, the same shape routes/billing.tsx uses.

Redirect before any collective lookup, for every slug: a signed-out visitor gets the same answer whether the collective exists, is private, or is personal, so the redirect reveals nothing a 404 would hide. After sign-in the existing handling applies unchanged:

• a personal collective redirects to /billing, query kept
• a plain member redirects to /o/<slug>
• a collective the viewer cannot see is a 404
• owners, admins and site admins see the billing page

Tests (tests/routes/billing_page_test.ts or a sibling)

• Signed-out visitor on /o/<slug>/billing of a public collective → 302 to login with returnUrl=/o/<slug>/billing
• Same for a private collective → 302 to login, not 404
• Same for a slug that does not exist → 302 to login
• Query string survives the redirect (e.g. ?license=taken)
• Signed-in owner, plain member and personal-collective cases behave exactly as today

Out of scope

/billing itself, the plan picker, and the Personal License form post. The CLI change lands separately in the swamp repo; this issue has no dependency on it.