Show estimated GIF size in the editor

[apples-kksk]

This adds a small GIF ~x MB estimate beside the export controls when GIF is selected.

The estimate samples up to the first 2 seconds of the selected range through the existing GIF conversion/compression path, then scales that size to the selected duration. If the editor options change, the previous in-flight sample is canceled so stale estimates do not pile up.

Checked:

• yarn build-main
• ./node_modules/.bin/tsc --project renderer/tsconfig.json --noEmit
• npx --yes -p node@16 -c 'yarn lint' (existing warnings only)\n- yarn test:main\n- npx --yes -p node@16 -c 'yarn build-renderer'\n\nCloses Show estimated GIF size in the editor window #172

[apples-kksk]

The CircleCI build did not reach the project steps. CircleCI rejected the current macOS executor before checkout with:\n\n\nJob was rejected because resource class m4pro.medium, image xcode:13.4.1 is not a valid resource class\n\n\nThat looks like the repository CircleCI image/resource-class configuration rather than a failure in this change. The local checks listed in the PR body completed successfully.