chore(auth): redirect sign-in/sign-up to app.cameleer.io

Both auth flows now navigate to the app domain rather than the
auth.cameleer.io subdomain:

  PUBLIC_AUTH_SIGNIN_URL → https://app.cameleer.io/sign-in
  PUBLIC_AUTH_SIGNUP_URL → https://app.cameleer.io/sign-in?first_screen=register

Updated:
- .env.example (the canonical reference values)
- OPERATOR-CHECKLIST.md (deploy-time secret values)
- src/config/auth.test.ts (test fixtures)
- src/middleware.ts (CSP-comment about <a> navigation target)
- src/pages/privacy.astro (visitor-facing external-links section
  in §6 of the privacy policy)

The auth.ts validator stays strict-https — the new URLs are still
absolute https URLs, just on a different host.  Logto itself may
still run at auth.cameleer.io as the OIDC backend; only the
visitor-facing /sign-in entry point moved.

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

src/pages/privacy.astro

@@ -67,7 +67,7 @@ const lastUpdated = '2026-04-24';
     <section class="mb-10">
       <h2 class="text-lg font-bold text-text mb-3">6. External links</h2>
       <p class="text-text-muted leading-relaxed">
-        Sign-in and sign-up links on this site navigate you to <span class="font-mono text-accent">auth.cameleer.io</span> (Logto identity service) and subsequently <span class="font-mono text-accent">platform.cameleer.io</span>. Those services have their own privacy policies, which apply from the moment you arrive there.
+        Sign-in and sign-up links on this site navigate you to <span class="font-mono text-accent">app.cameleer.io</span> (the Cameleer app, where authentication is handled by Logto). That service has its own privacy policy, which applies from the moment you arrive there.
       </p>
     </section>