feat: custom Logto image + auto-redirect to sign-in

- Add docker/logto.Dockerfile: builds custom Logto image with sign-in
  UI baked into /etc/logto/packages/experience/dist/
- Remove sign-in-ui init container, signinui volume, CUSTOM_UI_PATH
  (CUSTOM_UI_PATH is Logto Cloud only, not available in OSS)
- Remove sign-in build stage from SaaS Dockerfile (now in logto.Dockerfile)
- Remove docker/saas-entrypoint.sh (no longer needed)
- LoginPage auto-redirects to Logto OIDC on mount instead of showing
  "Sign in with Logto" button — seamless sign-in experience

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

Dockerfile

@@ -9,15 +9,6 @@ RUN echo "//gitea.siegeln.net/api/packages/cameleer/npm/:_authToken=${REGISTRY_T
 COPY ui/ .
 RUN npm run build
 
-# Sign-in UI: custom Logto sign-in experience
-FROM --platform=$BUILDPLATFORM node:22-alpine AS sign-in-frontend
-ARG REGISTRY_TOKEN
-WORKDIR /ui
-COPY ui/sign-in/package.json ui/sign-in/package-lock.json ui/sign-in/.npmrc ./
-RUN echo "//gitea.siegeln.net/api/packages/cameleer/npm/:_authToken=${REGISTRY_TOKEN}" >> .npmrc && npm ci
-COPY ui/sign-in/ .
-RUN npm run build
-
 # Maven build: runs natively on build host (no QEMU emulation)
 FROM --platform=$BUILDPLATFORM eclipse-temurin:21-jdk-alpine AS build
 WORKDIR /build
@@ -34,7 +25,6 @@ FROM eclipse-temurin:21-jre-alpine
 WORKDIR /app
 RUN addgroup -S cameleer && adduser -S cameleer -G cameleer
 COPY --from=build /build/target/*.jar app.jar
-COPY --from=sign-in-frontend /ui/dist/ /app/sign-in-dist/
 USER cameleer
 EXPOSE 8080
 ENTRYPOINT ["java", "-jar", "app.jar"]