feat translator: add transcription translation hotkey via OpenRouter + translate target language

[Alexintosh]

This PR adds an optional “translate after transcription” flow powered by an LLM (OpenRouter), alongside a target-language selector.

What changed

• Added a new shortcut binding: Transcribe with Translation (transcribe_with_translation).
• Added a new setting: Translate To (translate_target_language).
• Implemented translation post-action using OpenRouter (reuses existing Post-Process provider config: post_process_api_keys["openrouter"] and post_process_models["openrouter"], with a fallback model when unset).
• Added ${translate_target_language} variable support in Post Process prompt templates (alongside ${output}).
• Added overlay state "translating" so the overlay shows “Translating…” during the OpenRouter request.

How to configure / use

• Set Translate To in settings.
• Configure OpenRouter in the existing Post Process settings (behind the Post-Processing toggle), then use the Translate hotkey.

Testing

• Manual testing (macOS): verified translation hotkey works once OpenRouter key/model are configured; verified overlay shows “Translating…” during the API call.
• Automated:
• bun run lint
• bun run format:check
• bun run build
• cargo +1.88.0 test
• cargo +1.88.0 clippy --all-targets (passes; warnings exist in repo)

AI Assistance Disclosure

• AI used: Yes
• Extent: helped design and implement the feature end-to-end

[cjpais]

#355 was merged yesterday, I think this should suffice for the time being I don't want to add many shortcuts without better UI to support all the features people want.

[Alexintosh]

sounds good, personally I'd love to have a shortcut for each one of the postprocessing prompts.

[cjpais]

I understand, UI needs to be thought out first