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-compose.yml

@@ -64,11 +64,14 @@ services:
       postgres:
         condition: service_healthy
     entrypoint: ["sh", "-c", "npm run cli db seed -- --swe && npm start"]
+    volumes:
+      - signinui:/etc/logto/custom-ui
     environment:
       DB_URL: postgres://${POSTGRES_USER:-cameleer}:${POSTGRES_PASSWORD:-cameleer_dev}@postgres:5432/logto
       ENDPOINT: ${PUBLIC_PROTOCOL:-https}://${PUBLIC_HOST:-localhost}
       ADMIN_ENDPOINT: http://${PUBLIC_HOST:-localhost}:3002
       TRUST_PROXY_HEADER: 1
+      CUSTOM_UI_PATH: /etc/logto/custom-ui
     healthcheck:
       test: ["CMD-SHELL", "node -e \"require('http').get('http://localhost:3001/oidc/.well-known/openid-configuration', r => process.exit(r.statusCode === 200 ? 0 : 1)).on('error', () => process.exit(1))\""]
       interval: 5s
@@ -130,6 +133,7 @@ services:
       - /var/run/docker.sock:/var/run/docker.sock
       - jardata:/data/jars
       - bootstrapdata:/data/bootstrap:ro
+      - signinui:/data/sign-in-ui
     environment:
       SPRING_DATASOURCE_URL: jdbc:postgresql://postgres:5432/${POSTGRES_DB:-cameleer_saas}
       SPRING_DATASOURCE_USERNAME: ${POSTGRES_USER:-cameleer}
@@ -238,3 +242,4 @@ volumes:
   certs:
   jardata:
   bootstrapdata:
+  signinui: