fix: enable multilingual transcription (Russian, auto-detect)

[Nibomb]

Summary

Push-to-talk transcription is currently hardcoded to English ("en"), making it completely unusable for non-English speakers. This PR fixes the issue by:

• Defaulting to "multi" (multilingual auto-detect) instead of "en" — DeepGram Nova-3 supports automatic language detection for 10 languages including Russian, English, Spanish, French, German, Hindi, Portuguese, Japanese, Italian, and Dutch
• Persisting language settings to UserDefaults — previously the language reset to "en" on every app restart since transcriptionLanguage had no didSet persistence (unlike transcriptionVocabulary which already persists)
• Wiring up transcriptionAutoDetect — the property existed but was completely inert; now effectiveTranscriptionLanguage returns "multi" when auto-detect is enabled
• Expanding supportedLanguages — added "Auto-detect (Multilingual)", Russian, Ukrainian, Dutch, Polish, and Turkish to the language list
• Conditionally applying English-only replacements — find-and-replace rules like "dot com" → ".com" are now only sent to DeepGram when language is "en", preventing potential interference with non-English transcription

Context

The entire transcription pipeline (AssistantSettings → PushToTalkManager → TranscriptionService → DeepGram WebSocket) is fully functional — it just always received "en". Russian ("ru") was even listed in supportedLanguages but never surfaced to users via a language picker UI.

DeepGram Nova-3 fully supports Russian both as monolingual (language=ru) and via multilingual code-switching (language=multi), confirmed in their Nov 2025 announcement.

Test plan

• Start Fazm, open Push-to-talk settings
• Verify language defaults to "Auto-detect (Multilingual)" for new installs
• Test PTT with Russian speech — should transcribe correctly
• Test PTT with English speech — should still work
• Test switching between specific languages (e.g., Russian, English)
• Restart app — verify language setting persists
• Test batch transcription mode with non-English speech

Files changed

• Desktop/Sources/DeletedTypeStubs.swift — AssistantSettings: default language, persistence, auto-detect logic, expanded language list
• Desktop/Sources/TranscriptionService.swift — conditional English-only replacements in both streaming and batch modes

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