Fix ToImgopts/DownloadImgopts type definitions

[hikmetba-bit]

Summary

Fixes #8057.

Both ToImgopts and DownloadImgopts required format/width/height (and DownloadImgopts also required filename), but the runtime defaults every one of them:

• src/plot_api/to_image.js: opts = opts || {} — toImage()/downloadImage() can be called with no options object at all. format defaults to 'png', scale to 1, imageDataOnly to false; width/height fall back to the graph's own layout dimensions when omitted.
• src/snapshot/download.js: explicitly sets opts.format = opts.format || 'png', opts.width = opts.width || null, opts.height = opts.height || null before use, and derives filename from the graph title/subtitle (falling back to 'plot-image') when not given.

Also found two gaps against the runtime's shared attrs object in to_image.js while tracing this:

• format's value union was missing 'full-json' (valType: 'enumerated', values: [..., 'full-json'], dflt: 'png' — an exportable format since v1.53.0, per the issue).
• imageDataOnly (valType: 'boolean', dflt: false) had no type at all.

Added setBackground to both interfaces too: to_image.js's shared attrs object accepts it (valType: 'any'), and it's the same per-call override already typed for the top-level Config#setBackground in this same file ('opaque' | 'transparent' | ((gd, bgColor) => void)) — reused that exact type for consistency.

Testing

• npm run typecheck (tsc --noEmit, project-wide): clean, both before and after this change (no existing test exercised these two interfaces).
• Wrote a standalone positive-verification file exercising the fixed cases ({} for both interfaces, imageDataOnly: true, format: 'full-json', setBackground: 'opaque') and typechecked it directly with tsc --strict: passes cleanly against this change.
• Confirmed the same file fails to compile against the original config.d.ts with exactly the errors the issue describes (TS2739 missing required properties, TS2353 unknown property imageDataOnly, TS2322 'full-json' not assignable) — mutation-tested by stashing just this file.

🤖 Generated with Claude Code

[camdecoster]

We haven't decided on an approach yet, so I'm closing this PR. You're welcome to join the discussion in the issue.