Show representative GIF size estimates

[jamilahmadzai]

Summary

• show an approximate GIF size beside the Convert button
• convert the complete selection when it is at most three seconds, otherwise sample one second each from the start, middle, and end
• debounce option changes and cancel stale background conversions
• avoid interactive edit services and export analytics during estimates
• clean up generated samples and GIF palettes on success, failure, and cancellation
• replace the retired CircleCI macOS image with the Node 16 setup already proven by Show estimated GIF size in the editor #1277

Why this approach

A single sample from the beginning can be badly skewed when motion or visual complexity changes later in a recording. Distributing a bounded three-second budget across the selected clip remains predictable while producing a more representative estimate.

Validation

• full lint suite passes (18 pre-existing warnings)
• all 32 main-process tests pass
• main and renderer TypeScript checks pass
• production main build, Next.js build, and static export pass
• integration tests compare a short-selection estimate against an actual GIF and convert distributed samples for longer selections
• upstream CircleCI build passes, including the full test suite and x64/arm64 DMG packaging

CI note

The base branch's CircleCI job is rejected before checkout because the Xcode 13.4.1 runner combination is no longer valid. The second commit carries over the current macOS/Node 16 setup from #1277; the resulting upstream build completed successfully.

Closes #172