Add OpenAI cloud speech transcription

[Jinwoo-H]

Summary

• add OpenAI GPT-4o transcription model options to Voice settings
• store the OpenAI speech API key locally with Electron safeStorage when available
• route OpenAI speech dictation through the cloud transcription client and sanitize provider errors before showing toasts
• keep OpenAI key configuration progressively disclosed under the Speech Model setting

Closes #4634

Testing

• pnpm exec vitest run --config config/vitest.config.ts src/main/speech/model-manager.test.ts src/main/speech/stt-service.test.ts src/main/speech/openai-transcription-client.test.ts src/main/ipc/speech.test.ts src/renderer/src/components/settings/VoicePane.test.tsx
• pnpm exec oxlint src/main/ipc/speech.ts src/main/ipc/speech.test.ts src/main/speech/model-catalog.ts src/main/speech/model-manager.ts src/main/speech/model-manager.test.ts src/main/speech/stt-service.ts src/main/speech/stt-service.test.ts src/main/speech/openai-api-key-store.ts src/main/speech/openai-transcription-client.ts src/main/speech/openai-transcription-client.test.ts src/preload/api-types.ts src/preload/index.ts src/renderer/src/components/dictation/DictationController.tsx src/renderer/src/components/settings/VoicePane.tsx src/renderer/src/components/settings/VoicePane.test.tsx src/renderer/src/components/settings/OpenAiTranscriptionKeyDialog.tsx src/renderer/src/components/settings/OpenAiTranscriptionSettingsRow.tsx src/renderer/src/components/settings/voice-dictation-toggle.ts src/renderer/src/components/settings/voice-pane-search.ts src/shared/constants.ts src/shared/speech-types.ts
• pnpm run typecheck:node
• pnpm run typecheck:web
• git diff --check

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