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/Dockerfile

@@ -22,6 +22,7 @@ ARG VITE_GOOGLE_CLIENT_ID
 ARG VITE_MS_CLIENT_ID
 ARG VITE_OAUTH_REDIRECT_BASE
 ARG VITE_SELF_SERVE_ENABLED
+ARG VITE_CALENDLY_URL
 ENV VITE_API_URL=$VITE_API_URL
 ENV VITE_SENTRY_DSN=$VITE_SENTRY_DSN
 ENV VITE_PUBLIC_POSTHOG_KEY=$VITE_PUBLIC_POSTHOG_KEY
@@ -31,6 +32,7 @@ ENV VITE_GOOGLE_CLIENT_ID=$VITE_GOOGLE_CLIENT_ID
 ENV VITE_MS_CLIENT_ID=$VITE_MS_CLIENT_ID
 ENV VITE_OAUTH_REDIRECT_BASE=$VITE_OAUTH_REDIRECT_BASE
 ENV VITE_SELF_SERVE_ENABLED=$VITE_SELF_SERVE_ENABLED
+ENV VITE_CALENDLY_URL=$VITE_CALENDLY_URL
 
 # Build the application
 RUN npm run build