feat: show download state for local translation models

[Pewpenguin]

Summary

Local translation models now expose their per-quantization download state and size from the on-disk Hugging Face cache.

Downloaded quantizations show a Downloaded badge. Missing quantizations show the remaining download size and a Download & Load action, which saves the selected ModelSelection and loads it through the existing translator path. Download state is derived locally from the pinned model catalog, so opening the picker does not require a Hugging Face metadata request.

Normal model selection remains preference-only, and remote providers are unchanged. The Download & Load action is available in the editor model picker; Settings remains selection-only and shows download state/size without triggering a download.

Related issue

Closes #233

Verification

• cargo fmt -- --check
• cargo check -p koharu-app
• cargo test -p koharu-runtime --lib source::hugging_face
• cargo test -p koharu-translator --lib
• cargo test -p koharu-pipeline --lib tests::load_translation_model_is_noop_for_remote_providers
• bun run --filter @koharu/app lint
• bunx tsc --noEmit in packages/koharu
• Vitest: 15 test files, 115 tests passed
• git diff --check
• Manually verified downloaded and undownloaded quantizations, persistence after reopening the model picker, and the Download & Load flow.

UI

Downloaded quantization:

Settings:

Editor:

Undownloaded quantization:

Settings:

Editor:

AI assistance

Used AI assistance to understand the existing model-loading, Hugging Face cache, and frontend flows while developing the change. I reviewed the full diff, ran the checks above, and take responsibility for the change.

[github-actions]

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• how you verified the change

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[mayocream]

Please make minimal changes needed, only change the model list api to include a downloaded bool to indicate the status; let's use an icon for downloaded status instead of showing a literal.

[Pewpenguin]

Please make minimal changes needed, only change the model list api to include a downloaded bool to indicate the status; let's use an icon for downloaded status instead of showing a literal.

Addressed this point and kept the changes minimal. I used the HardDrive icon for the downloaded status — does that look appropriate, or would you prefer a different icon?