enhance lesson generation and focus mode functionality

server/api/topics/[id]/generate.post.ts

@@ -1,6 +1,6 @@
 import { db } from "../../../db/index";
 import { topics, lessons } from "../../../db/schema";
-import { eq } from "drizzle-orm";
+import { eq, and } from "drizzle-orm";
 import { generateLesson } from "../../../utils/generateLesson";
 
 export default defineEventHandler(async (event) => {
@@ -19,6 +19,18 @@ export default defineEventHandler(async (event) => {
 
   try {
     await generateLesson(id);
+
+    // pre-generate the next topic silently
+    const currentTopic = await db.query.topics.findFirst({ where: eq(topics.id, id) });
+    const nextTopic = await db.query.topics.findFirst({
+      where: and(
+        eq(topics.courseId, currentTopic!.courseId),
+        eq(topics.order, currentTopic!.order + 1),
+        eq(topics.status, "pending")
+      )
+    });
+    if (nextTopic) generateLesson(nextTopic.id);
+
     return { status: "ready" };
   } catch (err: any) {
     console.error(`[generate.post] topic ${id} failed: ${err?.message ?? err}`);