Support `Smart Split`

One-click “Smart Split” button

Pattern: a button appears when overflow is detected:

[Smart split into 2 slides]
[Smart split into 3 slides]
[Keep single slide (I’ll edit)]

The “smart split” algorithm can:

• Respect block boundaries only (never cut mid-sentence).

• Aim for ~80–90% height usage per slide (not razor-thin).

After splitting, present the new slides with a short toast:

Split into 3 slides. You can still move blocks between them.