Add cancellable GIF size estimates

[landeqiming666]

Summary

Adds an approximate GIF size next to the editor export controls. The estimate converts a short sample with the same dimensions, frame rate, and selected range, then scales it to the full duration.

The estimate is canceled when export options change, the format changes, or the editor component is unmounted, so an obsolete background conversion does not continue after the result can no longer be shown. The probe deliberately does not invoke edit services because those can be interactive or have side effects.

Validation

• TS_NODE_TRANSPILE_ONLY=1 node ./node_modules/ava/cli.js
• node ./node_modules/typescript/bin/tsc --noEmit -p tsconfig.json
• node ./node_modules/typescript/bin/tsc --noEmit -p renderer/tsconfig.json
• node ./node_modules/next/dist/bin/next build renderer
• node ./node_modules/next/dist/bin/next export renderer
• targeted XO lint
• git diff --check

Closes #172

[landeqiming666]

CircleCI did not reach checkout or run this change. The job was rejected because the repository macOS executor combines m4pro.medium with xcode:13.4.1, which CircleCI now reports as invalid.

Local validation completed: full AVA suite, both TypeScript projects, focused lint, production renderer build/export, and git diff --check.

[albinekb]

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