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>

docker/logto.Dockerfile

@@ -0,0 +1,14 @@
+# syntax=docker/dockerfile:1
+
+# Build custom sign-in UI
+FROM --platform=$BUILDPLATFORM node:22-alpine AS sign-in
+ARG REGISTRY_TOKEN
+WORKDIR /ui
+COPY ui/sign-in/package.json ui/sign-in/package-lock.json ui/sign-in/.npmrc ./
+RUN echo "//gitea.siegeln.net/api/packages/cameleer/npm/:_authToken=${REGISTRY_TOKEN}" >> .npmrc && npm ci
+COPY ui/sign-in/ .
+RUN npm run build
+
+# Custom Logto with baked-in sign-in UI
+FROM ghcr.io/logto-io/logto:latest
+COPY --from=sign-in /ui/dist/ /etc/logto/packages/experience/dist/