feat(litellm): add UI_USERNAME/UI_PASSWORD for the admin UI login

Without these, LiteLLM's /ui falls back to username 'admin' and the
master key as the password, forcing the master key to be typed into
a browser. generate-secrets.sh now fills UI_PASSWORD like the other
secrets.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q2CR8yawSf7pwAVYnjwFea

scripts/generate-secrets.sh

@@ -1,7 +1,8 @@
 #!/usr/bin/env bash
 # Generates random values for the secrets docker-compose.yml requires
-# (LITELLM_MASTER_KEY, LITELLM_SALT_KEY, LITELLM_DB_PASSWORD) and writes
-# them into .env — creating it from .env.example first if it doesn't exist.
+# (LITELLM_MASTER_KEY, LITELLM_SALT_KEY, LITELLM_DB_PASSWORD, UI_PASSWORD)
+# and writes them into .env — creating it from .env.example first if it
+# doesn't exist.
 #
 # ponytail: only fills in blank values, never overwrites ones you've already
 # set — safe to re-run. Re-running won't touch LITELLM_SALT_KEY once it's
@@ -24,5 +25,6 @@ set_if_blank() {
 set_if_blank LITELLM_MASTER_KEY "$(openssl rand -hex 32)"
 set_if_blank LITELLM_SALT_KEY "$(openssl rand -hex 32)"
 set_if_blank LITELLM_DB_PASSWORD "$(openssl rand -hex 32)"
+set_if_blank UI_PASSWORD "$(openssl rand -hex 16)"
 
 echo "Done. Review .env, then set OPENWEBUI_LITELLM_KEY per docs/proxy-key-onboarding.md."