feat: custom Logto image + auto-redirect to sign-in

- Add docker/logto.Dockerfile: builds custom Logto image with sign-in
  UI baked into /etc/logto/packages/experience/dist/
- Remove sign-in-ui init container, signinui volume, CUSTOM_UI_PATH
  (CUSTOM_UI_PATH is Logto Cloud only, not available in OSS)
- Remove sign-in build stage from SaaS Dockerfile (now in logto.Dockerfile)
- Remove docker/saas-entrypoint.sh (no longer needed)
- LoginPage auto-redirects to Logto OIDC on mount instead of showing
  "Sign in with Logto" button — seamless sign-in experience

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/auth/LoginPage.tsx

@@ -1,11 +1,12 @@
-import { useEffect } from 'react';
+import { useEffect, useRef } from 'react';
 import { useLogto } from '@logto/react';
 import { useNavigate } from 'react-router';
-import { Button, Spinner } from '@cameleer/design-system';
+import { Spinner } from '@cameleer/design-system';
 
 export function LoginPage() {
   const { signIn, isAuthenticated, isLoading } = useLogto();
   const navigate = useNavigate();
+  const redirected = useRef(false);
 
   useEffect(() => {
     if (isAuthenticated) {
@@ -13,27 +14,16 @@ export function LoginPage() {
     }
   }, [isAuthenticated, navigate]);
 
-  if (isLoading) {
-    return (
-      <div style={{ display: 'flex', alignItems: 'center', justifyContent: 'center', minHeight: '100vh' }}>
-        <Spinner />
-      </div>
-    );
-  }
-
-  const handleLogin = () => {
-    signIn(`${window.location.origin}/platform/callback`);
-  };
+  useEffect(() => {
+    if (!isLoading && !isAuthenticated && !redirected.current) {
+      redirected.current = true;
+      signIn(`${window.location.origin}/platform/callback`);
+    }
+  }, [isLoading, isAuthenticated, signIn]);
 
   return (
     <div style={{ display: 'flex', alignItems: 'center', justifyContent: 'center', minHeight: '100vh' }}>
-      <div style={{ textAlign: 'center' }}>
-        <h1>Cameleer SaaS</h1>
-        <p style={{ marginBottom: '2rem', color: 'var(--color-text-secondary)' }}>
-          Managed Apache Camel Runtime
-        </p>
-        <Button onClick={handleLogin}>Sign in with Logto</Button>
-      </div>
+      <Spinner />
     </div>
   );
 }