refactor(speech): adopt WebRTC VAD and official Google STT client

[TheShigure7]

#288
#289

[codecov]

Codecov Report

❌ Patch coverage is 47.20497% with 170 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/speech/vad_webrtc.go	14.11%	68 Missing and 5 partials ⚠️
pkg/speech/stt_openai_client.go	43.54%	22 Missing and 13 partials ⚠️
pkg/speech/stt_google.go	72.35%	28 Missing and 6 partials ⚠️
pkg/speech/vad_provider.go	63.15%	4 Missing and 3 partials ⚠️
pkg/speech/stt_google_client.go	33.33%	5 Missing and 1 partial ⚠️
pkg/speech/vad.go	14.28%	5 Missing and 1 partial ⚠️
pkg/speech/voicewake.go	40.00%	4 Missing and 2 partials ⚠️
pkg/speech/stt_provider.go	0.00%	1 Missing and 1 partial ⚠️
pkg/speech/stt_whisper.go	80.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[fennoai]

Found 3 noteworthy issues after code quality, performance, security, and documentation passes.