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/tsconfig.json

@@ -0,0 +1,21 @@
+{
+  "compilerOptions": {
+    "target": "ES2020",
+    "useDefineForClassFields": true,
+    "lib": ["ES2020", "DOM", "DOM.Iterable"],
+    "module": "ESNext",
+    "skipLibCheck": true,
+    "moduleResolution": "bundler",
+    "allowImportingTsExtensions": true,
+    "isolatedModules": true,
+    "moduleDetection": "force",
+    "noEmit": true,
+    "jsx": "react-jsx",
+    "strict": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "noFallthroughCasesInSwitch": true,
+    "noUncheckedSideEffectImports": true
+  },
+  "include": ["src"]
+}