feat(arr): add the OpenAI endpoint fields to /settings

Shown only when openai is among available_engines. Placeholders say the
base URL is optional and what a local one looks like.

web/src/settings.ts

@@ -46,6 +46,15 @@ export interface SubtitleSettings {
   provider_daily_budgets: Record<string, number>;
   translator_daily_budgets: Record<string, number>;
   remote_command_timeout_seconds: number;
+  /**
+   * Where the OpenAI-compatible backend points and which model it names
+   * (§15, #220). `null` on either means the backend's own default — that
+   * backend is anything speaking the shape, `llama.cpp` included, so both
+   * are settings rather than deployment facts. Neither is a secret; the API
+   * key stays in the environment.
+   */
+  openai_base_url: string | null;
+  openai_model: string | null;
   /** Engines this binary compiled in — `translation_engine` is always one of these, or null. */
   available_engines: string[];
 }
@@ -815,6 +824,20 @@ export function settingsMain(views: { hide: () => void }[], goHome: () => void):
     const timeout = numberControl(settings.remote_command_timeout_seconds);
     grid.append(field("remote command timeout (s)", timeout));
 
+    // #220: only meaningful when this build compiled the backend in, and the
+    // engine list is exactly that fact. Placeholders carry the two things
+    // the labels cannot: that the base URL is optional, and what a local one
+    // looks like.
+    const hasOpenAi = settings.available_engines.includes("openai");
+    const openAiBaseUrl = textControl(
+      settings.openai_base_url ?? "",
+      "optional — e.g. http://127.0.0.1:8080/v1",
+    );
+    const openAiModel = textControl(settings.openai_model ?? "", "optional — e.g. gpt-4o-mini");
+    if (hasOpenAi) {
+      grid.append(field("openai base url", openAiBaseUrl), field("openai model", openAiModel));
+    }
+
     panel.append(grid);
     if (!hasEngines) {
       panel.append(
@@ -908,6 +931,10 @@ export function settingsMain(views: { hide: () => void }[], goHome: () => void):
         provider_daily_budgets: providerBudgets,
         translator_daily_budgets: translatorBudgets,
         remote_command_timeout_seconds: Number(timeout.value),
+        // An emptied field is "use the backend's default", which the API
+        // stores as NULL.
+        openai_base_url: hasOpenAi ? openAiBaseUrl.value.trim() || null : settings.openai_base_url,
+        openai_model: hasOpenAi ? openAiModel.value.trim() || null : settings.openai_model,
       };
       void saveSubtitleSettings(input).then((outcome) => {
         if (outcome.ok) {