Build and browser-verify the tenant-picker frontend page

web/src/routes/select-tenant now calls enterprise-auth's existing
GET /auth/memberships / POST /auth/select-tenant protocol (built earlier
this phase, previously called only from Go tests) via
fetch(..., {credentials: 'include'}) -- new listMemberships/selectTenant
functions in $lib/api.ts, using a dedicated request helper that reads
plain-text error bodies (loginhandler's http.Error responses), unlike
every other request helper in that file which expects JSON.

Credentialed cross-origin fetch needed CORS enterprise-auth didn't have:
api/httpserver.WithCORS's wildcard-friendly default can't be combined
with a credentialed request at all (browsers refuse to honor
Access-Control-Allow-Origin: "*" on one) -- added WithCredentialedCORS
(literal origin, Access-Control-Allow-Credentials: true) alongside it,
wired into enterprise-auth via a new CORS_ALLOWED_ORIGIN config var
defaulting to POST_LOGIN_REDIRECT_URL (web's own origin, the same
default pattern SELECT_TENANT_REDIRECT_URL already used).

adapter-static's route crawler doesn't discover a page nothing links to
(this one is only ever reached via enterprise-auth's redirect) -- fixed
with select-tenant/+page.ts's `export const prerender = true`, the same
declaration every other route already has.

Genuinely verified in a real browser in this environment, not just
type-checked: a throwaway Node server standing in for enterprise-auth's
exact wire contract (including its plain-text error bodies), driven
through the full flow via mcp__claude-in-chrome -- cross-origin
pending-login cookie set, credentialed preflight + GET/POST round trip,
a real click choosing a tenant, the post-selection redirect, and the
missing/expired-cookie error path rendering the backend's actual
message. No Docker or live Postgres/IdP needed, since the point was
exercising web's own fetch/CORS/cookie wiring, not enterprise-auth's
internals (already covered by loginhandler's own tests).

This closes the tenant-picker as the last named gap in Phase 4. What's
left is the already-disclosed live-verification caveat shared by every
Postgres/ClickHouse-backed piece and both SSO protocols: none of this
has run against a real database, external IdP, or multi-container
deployment in this environment.

api/httpserver/cors.go

@@ -22,3 +22,29 @@ func WithCORS(next http.Handler, allowedOrigin string) http.Handler {
 		next.ServeHTTP(w, r)
 	})
 }
+
+// WithCredentialedCORS is WithCORS's sibling for endpoints a browser must
+// call with cookies attached (Phase 4: enterprise-auth's
+// GET /auth/memberships / POST /auth/select-tenant, called from web's
+// tenant-picker page via `fetch(..., {credentials: 'include'})`).
+// Browsers categorically refuse to combine a credentialed request with
+// Access-Control-Allow-Origin: "*" -- allowedOrigin must be a real,
+// literal origin (e.g. "http://localhost:3000"), not the wildcard
+// WithCORS's own zero-config default relies on. Callers of this function
+// don't get that convenient zero-config default: an empty/wildcard
+// allowedOrigin here is a configuration bug, not a permissive default,
+// so it's deliberately not special-cased into something that "just
+// works" the way plain WithCORS's "*" does.
+func WithCredentialedCORS(next http.Handler, allowedOrigin string) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Access-Control-Allow-Origin", allowedOrigin)
+		w.Header().Set("Access-Control-Allow-Credentials", "true")
+		w.Header().Set("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS")
+		w.Header().Set("Access-Control-Allow-Headers", "Content-Type")
+		if r.Method == http.MethodOptions {
+			w.WriteHeader(http.StatusNoContent)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}