Pretranslation — Preview & Progress

Toucan's pretranslation engine now supports safe preview runs with progress reporting and an explicit commit step.

Key behaviors:

• PreviewOnly mode: Providers can be asked to perform a dry-run (PreviewOnly) — the result is returned as a list of proposed translations but the project files are not modified.
• Progress reporting: Pretranslation providers implement progress/reporting via IProgress<PretranslationProgress> so the UI can show completed/total counts and a progress-bar.
• Cancellation: Long-running preview runs or provider calls can be cancelled via a CancellationToken.
• Commit step: After previewing translations, the UI shows a summarized list and lets the user Apply Preview (commit) to update the local translations.

UI integration:

• The Pre-Translate window displays a progress bar while preview runs are in-flight and a list of proposed translations (namespace, language, source, proposed translation) once complete.
• An "Apply Preview" button lets the user apply those results to the in-memory translations; the user must explicitly click Apply for changes to be persisted.

Privacy note:

• Preview mode sends your source strings to the selected provider in order to obtain translations. Always be mindful of provider privacy policies when translating sensitive strings.