iPad image download bug for huge maps

[JannikStreek]

Describe the bug
When exporting a big map (meaning not many nodes but just nodes with huge distances), image download on the iPad adds a txt ending which makes image usage difficult.

To Reproduce
Steps to reproduce the behavior:

1. Create a big mind map
2. Download as image on the iPad
3. See txt ending of file

Expected behavior
No txt ending of the file

[sorenjohanson]

So the reason why Safari adds .txt at the end is that the image exported is actually 0KB. Chrome downloads it correctly, but it still doesn't show anything. Seems to be a general export bug.

[sorenjohanson]

Can be reproduced on Desktop as well.

[sorenjohanson]

This is caused by canvas.toDataUrl() in the export handler: https://github.com/b310-digital/teammapper/blob/main/teammapper-frontend/mmp/src/map/handlers/export.ts#L86

I'm assuming there's a practical character limit to the base64 string and we're just exceeding it.

[sorenjohanson]

We're hitting the practical limits of canvasses when exporting images:

Maximum height/width: 32,767 pixels
Maximum area: 268,435,456 pixels (e.g., 16,384 x 16,384)

I think the only feasible solution would be to simply limit the working area to 16,000 x 16,000 px (leaving a bit of buffer) and displaying the rest of the area in grey.