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.module.css

@@ -0,0 +1,58 @@
+.page {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  min-height: 100vh;
+  background: var(--bg-base);
+}
+
+.card {
+  width: 100%;
+  max-width: 400px;
+  padding: 32px;
+}
+
+.loginForm {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  font-family: var(--font-body);
+  width: 100%;
+}
+
+.logo {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  margin-bottom: 8px;
+  font-size: 24px;
+  font-weight: 700;
+  color: var(--text-primary);
+}
+
+.logoImg {
+  width: 36px;
+  height: 36px;
+}
+
+.subtitle {
+  font-size: 13px;
+  color: var(--text-muted);
+  margin: 0 0 24px;
+}
+
+.error {
+  width: 100%;
+  margin-bottom: 16px;
+}
+
+.fields {
+  display: flex;
+  flex-direction: column;
+  gap: 14px;
+  width: 100%;
+}
+
+.submitButton {
+  width: 100%;
+}