fix: grant admin role to anonymous user and add DEFAULT_TOOL_VIEW env var

When AUTH_ENABLED=false, the anonymous user was assigned the "user" role
which lacks settings:write permission, making all settings saves return
403. Since no admin exists when auth is disabled, settings were
permanently read-only. Promote the anonymous user to "admin" so the
single user has full control of the instance.

Also adds DEFAULT_TOOL_VIEW env var (sidebar|fullscreen) following the
existing DEFAULT_THEME pattern, seeded via ensureDefaultSettings() on
first boot.

Closes #135

docker/Dockerfile

@@ -255,6 +255,7 @@ ENV PORT=1349 \
     U2NET_HOME=/data/ai/models/rembg \
     DEFAULT_THEME=light \
     DEFAULT_LOCALE=en \
+    DEFAULT_TOOL_VIEW=sidebar \
     FILE_MAX_AGE_HOURS=72 \
     CLEANUP_INTERVAL_MINUTES=60 \
     MAX_UPLOAD_SIZE_MB=0 \