feat(byok): add model selection to BYOK settings panel and overlay custom model on route resolution

memento-note/lib/ai/provider-for-user.ts

@@ -25,6 +25,7 @@ async function resolveProviderForLane(
   const cfg = { ...config };
   const route = resolveAiRoute(lane, cfg);
   let usedByok = false;
+  let byokModel: string | null = null;
 
   if (billingUserId) {
     const overlay = await applyByokToConfig(
@@ -34,17 +35,22 @@ async function resolveProviderForLane(
     );
     Object.assign(cfg, overlay.config);
     usedByok = overlay.usedByok;
+    byokModel = overlay.model;
   }
 
+  const resolvedModel = byokModel && byokModel.trim() !== '' ? byokModel : route.modelName;
+
   const provider = getProviderInstance(
     route.providerType as ProviderType,
     cfg,
-    route.modelName,
+    resolvedModel,
     route.embeddingModelName,
     route.ollamaBaseUrl,
   );
 
-  return { provider, usedByok, route };
+  const updatedRoute = { ...route, modelName: resolvedModel };
+
+  return { provider, usedByok, route: updatedRoute };
 }
 
 async function getChatProviderForBillingUser(