Fix image and diagram overflow

[remiconnesson]

What changed

• fit split-image copy to the available slide frame
• add fit: cover | contain to image and showcase layouts
• constrain figures inside diagram stages
• document the safe authoring pattern for technical diagram images

Root cause

The image layout always used cover, so full-canvas screenshots and diagrams were cropped inside a side pane. Its text column also skipped Tahta's fit-to-frame wrapper, allowing long copy to extend below the slide.

Verification

• npm test from an isolated worktree containing only this commit
• real Slidev browser render with the failing 4056x2664 diagram asset
• verified contained image bounds, fitted copy bounds, and no browser error overlay