Set the OpenAI-compatible endpoint from the settings panel

Pointing OPENAI_BASE_URL at a local llama-server or Ollama sends every gpt-*
call there instead of to OpenAI, and litellm reports that as a bare
"Connection Error" — which reads like a network problem even though the host
can reach api.openai.com fine. Deploying a .env that still carries the local
endpoint reproduces it on the server, with nothing in the message naming the
culprit.

Add OPENAI_BASE_URL and OPENAI_API_BASE to the settings allowlist so the
endpoint can be corrected without editing .env and recreating the container.
Both are listed because litellm reads OPENAI_API_BASE while the OpenAI SDK
reads OPENAI_BASE_URL, so fixing one can leave the other in effect. Neither is
a secret, so the panel shows the current value.

No cache invalidation is needed alongside this: litellm keys its client cache
on the endpoint, so a changed value takes effect on the next call.

Document the trap where it will be read — a commented template in .env.example
and a note in the Lightsail deploy guide about carrying a local .env over.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/components/SettingsPanel.vue

@@ -37,6 +37,8 @@ async function addModelPreset() {
 
 const KEY_LABELS: Record<string, string> = {
   OPENAI_API_KEY: 'OpenAI APIキー',
+  OPENAI_BASE_URL: 'OpenAI 互換エンドポイント (OPENAI_BASE_URL)',
+  OPENAI_API_BASE: 'OpenAI 互換エンドポイント (OPENAI_API_BASE)',
   ANTHROPIC_API_KEY: 'Anthropic APIキー',
   AWS_ACCESS_KEY_ID: 'AWS Access Key ID',
   AWS_SECRET_ACCESS_KEY: 'AWS Secret Access Key',