feat(ai): split shared provider config from Anki runtime

src/ai/config.ts

@@ -0,0 +1,27 @@
+import type { AiConfig, AiFeatureConfig } from '../types';
+
+function trimToOverride(value: string | undefined): string | undefined {
+  if (typeof value !== 'string') return undefined;
+  const trimmed = value.trim();
+  return trimmed.length > 0 ? trimmed : undefined;
+}
+
+export function mergeAiConfig(
+  sharedConfig: AiConfig | undefined,
+  featureConfig?: AiFeatureConfig | boolean | null,
+): AiConfig {
+  const overrides =
+    featureConfig && typeof featureConfig === 'object' ? featureConfig : undefined;
+  const modelOverride = trimToOverride(overrides?.model);
+  const systemPromptOverride = trimToOverride(overrides?.systemPrompt);
+
+  return {
+    enabled: sharedConfig?.enabled,
+    apiKey: sharedConfig?.apiKey,
+    apiKeyCommand: sharedConfig?.apiKeyCommand,
+    baseUrl: sharedConfig?.baseUrl,
+    model: modelOverride ?? sharedConfig?.model,
+    systemPrompt: systemPromptOverride ?? sharedConfig?.systemPrompt,
+    requestTimeoutMs: sharedConfig?.requestTimeoutMs,
+  };
+}