feat: custom Logto sign-in UI with Cameleer branding

Replace Logto's default sign-in page with a custom React SPA that
matches the cameleer3-server login page using @cameleer/design-system.

- New Vite+React app at ui/sign-in/ with Experience API integration
- 4-step auth flow: init → verify password → identify → submit
- Design-system components: Card, Input, Button, FormField, Alert
- Same witty random subtitles as cameleer3-server LoginPage
- Dockerfile: add sign-in-frontend build stage, copy dist to image
- docker-compose: CUSTOM_UI_PATH on Logto, shared signinui volume
- SaaS entrypoint copies sign-in dist to shared volume on startup
- Add .gitattributes for LF line endings

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

ui/sign-in/src/SignInPage.tsx

@@ -0,0 +1,109 @@
+import { type FormEvent, useMemo, useState } from 'react';
+import { Card, Input, Button, Alert, FormField } from '@cameleer/design-system';
+import { signIn } from './experience-api';
+import styles from './SignInPage.module.css';
+
+const SUBTITLES = [
+  "Prove you're not a mirage",
+  "Only authorized cameleers beyond this dune",
+  "Halt, traveler — state your business",
+  "The caravan doesn't move without credentials",
+  "No hitchhikers on this caravan",
+  "This oasis requires a password",
+  "Camels remember faces. We use passwords.",
+  "You shall not pass... without logging in",
+  "The desert is vast. Your session has expired.",
+  "Another day, another dune to authenticate",
+  "Papers, please. The caravan master is watching.",
+  "Trust, but verify — ancient cameleer proverb",
+  "Even the Silk Road had checkpoints",
+  "Your camel is parked outside. Now identify yourself.",
+  "One does not simply walk into the dashboard",
+  "The sands shift, but your password shouldn't",
+  "Unauthorized access? In this economy?",
+  "Welcome back, weary traveler",
+  "The dashboard awaits on the other side of this dune",
+  "Keep calm and authenticate",
+  "Who goes there? Friend or rogue exchange?",
+  "Access denied looks the same in every desert",
+  "May your routes be green and your tokens valid",
+  "Forgot your password? That's between you and the dunes.",
+  "No ticket, no caravan",
+];
+
+export function SignInPage() {
+  const subtitle = useMemo(() => SUBTITLES[Math.floor(Math.random() * SUBTITLES.length)], []);
+  const [username, setUsername] = useState('');
+  const [password, setPassword] = useState('');
+  const [loading, setLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  const handleSubmit = async (e: FormEvent) => {
+    e.preventDefault();
+    setError(null);
+    setLoading(true);
+    try {
+      const redirectTo = await signIn(username, password);
+      window.location.replace(redirectTo);
+    } catch (err) {
+      setError(err instanceof Error ? err.message : 'Sign-in failed');
+      setLoading(false);
+    }
+  };
+
+  return (
+    <div className={styles.page}>
+      <Card className={styles.card}>
+        <div className={styles.loginForm}>
+          <div className={styles.logo}>
+            <img src="/platform/favicon.svg" alt="" className={styles.logoImg} />
+            cameleer3
+          </div>
+          <p className={styles.subtitle}>{subtitle}</p>
+
+          {error && (
+            <div className={styles.error}>
+              <Alert variant="error">{error}</Alert>
+            </div>
+          )}
+
+          <form className={styles.fields} onSubmit={handleSubmit} aria-label="Sign in" noValidate>
+            <FormField label="Username" htmlFor="login-username">
+              <Input
+                id="login-username"
+                value={username}
+                onChange={(e) => setUsername(e.target.value)}
+                placeholder="Enter your username"
+                autoFocus
+                autoComplete="username"
+                disabled={loading}
+              />
+            </FormField>
+
+            <FormField label="Password" htmlFor="login-password">
+              <Input
+                id="login-password"
+                type="password"
+                value={password}
+                onChange={(e) => setPassword(e.target.value)}
+                placeholder="••••••••"
+                autoComplete="current-password"
+                disabled={loading}
+              />
+            </FormField>
+
+            <Button
+              variant="primary"
+              type="submit"
+              loading={loading}
+              disabled={loading || !username || !password}
+              className={styles.submitButton}
+            >
+              Sign in
+            </Button>
+          </form>
+        </div>
+      </Card>
+    </div>
+  );
+}