fix(cli): implement generateThumbnail in studio adapter

The CLI adapter was missing generateThumbnail, so the thumbnail route
always returned 501 and composition thumbnails showed as blank black
rectangles in both the Compositions sidebar and the Timeline.

packages/cli/src/server/studioServer.ts

@@ -152,6 +152,47 @@ export function createStudioServer(options: StudioServerOptions): StudioServer {
 
       return state;
     },
+
+    async generateThumbnail(opts): Promise<Buffer | null> {
+      let instance: import("puppeteer-core").Browser | null = null;
+      try {
+        const { ensureBrowser } = await import("../browser/manager.js");
+        const browser = await ensureBrowser();
+        const puppeteer = await import("puppeteer-core");
+        instance = await puppeteer.launch({
+          executablePath: browser.executablePath,
+          headless: true,
+          args: ["--no-sandbox", "--disable-setuid-sandbox"],
+        });
+        const page = await instance.newPage();
+        await page.setViewport({ width: opts.width || 1920, height: opts.height || 1080 });
+        await page.goto(opts.previewUrl, { waitUntil: "networkidle2", timeout: 15000 });
+        await page
+          .waitForFunction(() => (window as any).__playerReady || (window as any).__timelines, {
+            timeout: 8000,
+          })
+          .catch(() => {});
+        await page.evaluate((t: number) => {
+          const win = window as any;
+          if (win.__player?.seek) win.__player.seek(t);
+          else if (win.__timeline?.seek) {
+            win.__timeline.pause();
+            win.__timeline.seek(t);
+          }
+        }, opts.seekTime);
+        await page.waitForTimeout(300);
+        const screenshot = (await page.screenshot({ type: "jpeg", quality: 85 })) as Buffer;
+        return screenshot;
+      } catch {
+        return null;
+      } finally {
+        try {
+          await instance?.close();
+        } catch {
+          /* ignore close errors */
+        }
+      }
+    },
   };
 
   // ── Build the Hono app ─────────────────────────────────────────────────