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>

docker/saas-entrypoint.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+# Copy sign-in UI dist to shared volume for Logto's CUSTOM_UI_PATH
+if [ -d /app/sign-in-dist ] && [ -d /data/sign-in-ui ]; then
+  cp -r /app/sign-in-dist/* /data/sign-in-ui/
+  echo "[saas] Copied sign-in UI to shared volume"
+fi
+exec java -jar /app/app.jar "$@"