feat: add local custom whisper stt

[tover0314-w]

Summary

• Add a Local / Custom Whisper STT provider with Speaches defaults (http://localhost:8000/v1, Systran/faster-whisper-large-v3) and optional API key support.
• Wire custom STT config through storage, pipeline setup, connection tests, and Settings UI while keeping onboarding on cloud/API-key providers.
• Add focused Rust and frontend coverage plus design/implementation docs for the feature.

Validation

• npm test (96 passed)
• npm run build
• git diff --check origin/main..HEAD
• CARGO_BUILD_JOBS=1 cargo test in src-tauri (77 passed)

Notes

Manual validation against a live local STT server was not run in this environment.