fix(scripts): pass rational fps to capture/render in catalog previews

`scripts/generate-catalog-previews.ts` still called `createCaptureSession`
with `fps: 30` and `createRenderJob` with `fps: 24`. Since commit 5dcc89c
("feat(cli): accept ffmpeg-style rational fps") `CaptureOptions.fps` and
`RenderConfig.fps` are `Fps = { num, den }` rationals — a plain number
yields `options.fps.den === undefined` and:

```ts
beginFrameIntervalMs: (1000 * options.fps.den) / Math.max(1, options.fps.num),
// = (1000 * undefined) / Math.max(1, undefined) = NaN / NaN = NaN
```

After warmup, `session.beginFrameTimeTicks = (baseTickCount + 10) * NaN = NaN`,
and the next `HeadlessExperimental.beginFrame` CDP call fails with:

```
Protocol error (HeadlessExperimental.beginFrame): Invalid parameters
Failed to deserialize params.frameTimeTicks - BINDINGS: double value expected
```

This regression didn't surface earlier because the Catalog Previews workflow
only re-renders items whose files changed in the PR, so existing components
were never exercised against the new fps contract. The vignette addition is
the first new item since the refactor.

Fix: pass `{ num: 30, den: 1 }` and `{ num: 24, den: 1 }`.

Author: jrusso1020

scripts/generate-catalog-previews.ts

@@ -243,7 +243,7 @@ async function generateThumbnail(item: CatalogItem, projectDir: string): Promise
     const session = await createCaptureSession(fileServer.url, framesDir, {
       width,
       height,
-      fps: 30,
+      fps: { num: 30, den: 1 },
       format: "png",
     });
     await initializeSession(session);
@@ -276,7 +276,7 @@ async function generateVideo(item: CatalogItem, projectDir: string): Promise<voi
 
   const outMp4 = join(outDir, `${item.name}.mp4`);
   const job = createRenderJob({
-    fps: 24,
+    fps: { num: 24, den: 1 },
     quality: "draft",
     format: "mp4",
   });