feat: download Whisper model automatically on startup

[hashbyr]

Summary

• Download the multilingual Whisper tiny model (~75 MB) in the background on desktop startup when no local model is installed.
• Reuse existing models, share in-flight downloads, resume interrupted transfers, and expose progress and retryable errors in Settings.
• Keep startup responsive when offline; settle cancellations and handle interrupted responses, timeouts, and resume responses safely.
• Integrate with current main while preserving Whisper vocabulary hints, large-turbo model support, English defaults, and Copilot CLI support. PRs feat: add GitHub Copilot CLI report provider #16 and Default interface to English with optional language selection #17 are already merged.

Validation

• 62 focused unit tests passed on the resolved merge.
• 6 Electron UI tests passed, including actual startup model download, restart reuse, vocabulary hint persistence, progress/error handling, and language switching.
• Desktop build, TypeScript typecheck, and changed-file ESLint passed.

Merged locally as a5214b7. Direct upstream push was rejected because the authenticated account lacks write permission; this PR contains the remaining startup-download change for maintainer merge.