Adjustable image export for question graphs

[ncarazon]

Context

Slack thread: https://metaculus.slack.com/archives/C01Q9AQBVHB/p1783025113081079

Feature description

• Make image export to utilize the same zoom value that is applied in question page
• Update embed modal to allow exporting image based on specified configuration (theme, zoom, graph type)

[aseckin]

@ncarazon I'll share an idea for the 2nd bullet point