Add UI language selector for transcription

[sammargolis]

Background

Multilingual transcription support is already implemented in backend/docker/docs via WHISPER_LANGUAGE (including optional auto-detect and continued .en compatibility).

This issue should build on that work by making language selection user-accessible in the UI, instead of env-only configuration.

Objective

Let users choose transcription language from the app UI and route that value through the existing transcription pipeline without changing current defaults.

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Scope

1. Add a transcription language selector in UI settings (or recording/transcription panel).

• Include Auto (default) option.
• Include a small set of common language options first (e.g., English, Spanish, French, etc.).

2. Wire selected value into existing transcription request/config path.

• Reuse current backend support for WHISPER_LANGUAGE.
• No model/engine changes required.

3. Persist user selection.

• Keep it across reload/restart (existing settings store/local persistence).

4. Keep current behavior unchanged when not set.

• Default remains auto/English-compatible existing flow.

Out of Scope

• New model integrations.
• Full locale/i18n for all UI text.
• Advanced language auto-routing heuristics.
• Major backend refactors.

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Detailed Success Criteria

1. Functional

• User can select transcription language from UI.
• Selection is applied to new transcription runs.
• Auto option works and maps to existing optional language behavior.

2. Backward compatibility

• English transcription remains unchanged when user keeps default.
• Existing .en model workflows still work.
• Docker/SAM/local workflows continue functioning.

3. Reliability

• Invalid/unsupported UI value is safely rejected or falls back to default.
• Clear UI/console error path for bad language config.

4. Persistence

• Selected language persists across app restart/reload.

5. Test coverage

• Add tests for:
  • default (Auto) behavior
  • explicit language selection (at least one non-English case path)
  • persistence and restore
  • fallback on invalid value

6. Documentation

• Add a short docs section:
  • how UI selection relates to WHISPER_LANGUAGE
  • precedence rules (UI override vs env default)

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Suggested Acceptance Checklist

• UI selector exists with Auto + language options
• Transcription requests use selected language
• Default behavior unchanged if user does nothing
• Existing multilingual backend flow still passes
• Docs updated with precedence + examples
• Tests added and passing

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Rationale

• Builds directly on the completed multilingual backend/env work.
• Small, high-impact UX improvement.
• Keeps implementation low-risk by reusing existing WHISPER_LANGUAGE support.