Add keynote perview support

[Ddawy]

现有格式没有keynote预览，希望增加该格式

[wybaby168]

Keynote support needs a separate offline feasibility review. Tracked in batch 4.

[wybaby168]

Modern .key files only expose a first-slide preview; full offline rendering still needs a real IWA engine. Keeping this open instead of shipping partial support.