Handle deleted images in translation editor

[floze-the-genius]

Summary

• reject non-successful and malformed image API responses before they reach the chooser render path
• show a local missing-image message instead of crashing the whole translation editor
• ignore stale async responses when the selected image changes
• add a dependency-free regression test and run frontend tests/builds in CI

Root cause

The chooser parsed every API response and stored it as image metadata. A deleted image returns a non-2xx error payload without thumbnail, so rendering imageInfo.thumbnail.url threw and unmounted the editor.

Fixes #769.

Validation

• npm test
• ./node_modules/.bin/tsc --noEmit
• NODE_OPTIONS=--openssl-legacy-provider npm run build
• Node 14.21.3 Docker gate: regression test, TypeScript compile, and production build
• Prettier 2.7.1 and the repository ESLint stack on changed frontend files
• git diff --check

AI disclosure

I used OpenAI Codex to help investigate the issue, implement the fix, and run validation. I reviewed the resulting code and test coverage before submission.