Expose gateway billing flows from the synced server baseline

The preserved local work adds PopiNewAPI billing checkout, gateway account binding, and matching web console/pricing surfaces on top of the current test-server baseline. During the merge, the start-end video helpers from the deployed baseline were kept alongside the actionGenerate prompt handling from the WIP.

Constraint: Current usable baseline is 7054436, already deployed on the test server.

Rejected: Commit the WIP before syncing the baseline | would have hidden conflicts with the deployed start-end-frame changes.

Confidence: medium

Scope-risk: broad

Directive: Do not deploy this commit to the test server without rechecking gateway credentials and billing checkout behavior in that environment.

Tested: go test ./...

Tested: make build

Tested: cd web && npm run build

web/components/main-nav.tsx

@@ -29,6 +29,7 @@ export function MainNav({
     skills: string;
     console: string;
     pricing: string;
+    billing?: string;
   };
 }) {
   const pathname = usePathname();
@@ -39,6 +40,9 @@ export function MainNav({
     { href: localize(locale, "/console"), label: labels.console },
     { href: localize(locale, "/pricing"), label: labels.pricing },
   ];
+  if (labels.billing) {
+    items.push({ href: localize(locale, "/billing"), label: labels.billing });
+  }
 
   return (
     <nav className="main-nav" aria-label="Primary">