feat(sales): add /contact-sales form + landing page CTA

Public Talk-to-Sales surface and a "See pricing" hero CTA on the marketing
landing page. Phase 2 Task 43 of self-serve signup.

- frontend/src/api/sales.ts: salesApi.createLead -> POST /sales-leads.
- ContactSalesPage at /contact-sales (public, gated by self_serve_enabled
  with a 404-style fallback). Form fields: name, work email, company,
  team size (1-2 / 3-5 / 6-10 / 11-25 / 26+), and an optional
  "what brought you here?" textarea -> message. Submit button disabled
  while in flight to block duplicate submissions.
- Confirmation surface replaces the form on success. Calendly block is
  hidden when VITE_CALENDLY_URL is unset.
- detectSource(): 'pricing_page' if document.referrer contains '/pricing',
  else 'landing_page'. Server emits the canonical PostHog
  talk_to_sales_form_submitted event with this source.
- LandingPage: new "See pricing" hero CTA gated by useAppConfig().
  self_serve_enabled.
- frontend/.env.example + Dockerfile: VITE_CALENDLY_URL ARG/ENV.
- Tests: ContactSalesPage submit/confirmation, Calendly hide-when-unset,
  in-flight de-dup, 404 when self-serve off; LandingPage CTA on/off.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/router.tsx

@@ -23,6 +23,7 @@ const SurveyThankYouPage = lazyWithRetry(() => import('@/pages/SurveyThankYouPag
 const PrivacyPage = lazyWithRetry(() => import('@/pages/PrivacyPage'))
 const TermsPage = lazyWithRetry(() => import('@/pages/TermsPage'))
 const PricingPage = lazyWithRetry(() => import('@/pages/PricingPage'))
+const ContactSalesPage = lazyWithRetry(() => import('@/pages/ContactSalesPage'))
 
 // Standalone auth pages
 const VerifyEmailPage = lazyWithRetry(() => import('@/pages/VerifyEmailPage'))
@@ -137,6 +138,11 @@ export const router = sentryCreateBrowserRouter([
     element: page(PricingPage),
     errorElement: <RouteError />,
   },
+  {
+    path: '/contact-sales',
+    element: page(ContactSalesPage),
+    errorElement: <RouteError />,
+  },
   {
     path: '/login',
     element: <LoginPage />,