## What
Add a script that uses `@hyperframes/producer` to render PNG thumbnails of each built-in template. Output goes to `docs/images/templates/`.
## Why
User feedback: "I would have loved more visual examples of what's actually possible. I had to scaffold every template just to see what they look like." This is the foundation for a visual template gallery in the docs.
## How
- New `scripts/generate-template-previews.ts` uses the producer's `createFileServer` + `createCaptureSession` + `captureFrame` APIs — hyperframes renders its own templates
- Patches out `__VIDEO_SRC__` placeholders (same logic as `init.ts`) so templates render without a video file
- Captures a frame at t=2s for each template (skips `blank` — it's just empty scaffolding)
- Handles varying dimensions (vignelli is 1080x1920 portrait)
- Adds `pnpm generate:previews` npm script
## Test plan
- [x] `pnpm generate:previews` generates 8 PNGs in `docs/images/templates/`
- [x] Each PNG is visually correct (verified by viewing)
- [x] `--only <template>` flag works for single template generation
