Wire custom community Slide.tsx renderers at runtime

## Summary
SDK scaffold creates `Slide.tsx` per layout, but runtime always uses `SlideCanvas` from `@slideforge/ui`.

## Tasks
- [ ] Load optional `Slide.tsx` from `community//` at API startup or on demand
- [ ] Fallback to `SlideCanvas` when no custom renderer
- [ ] Document when custom renderers are appropriate vs slot-only layouts
- [ ] Preview command uses same loader

## Files
- `packages/ui/src/SlideCanvas.tsx`
- `apps/api/src/services/layout-registry.ts`
- `packages/sdk/src/preview.ts`
- `community/README.md`

Read apps/api/src/services/layout-registry.ts first, then compare packages/sdk/src/preview.ts with packages/ui/src/SlideCanvas.tsx to understand the current rendering path. Done means optional community//Slide.tsx renderers load at API startup or on demand, missing renderers fall back to SlideCanvas, the preview command uses the same loader, and community/README.md documents when custom renderers are appropriate.

由索引模型根據 Issue 內容生成。