Hallucination fix + GPU acceleration

• trim trailing silence before whisper to prevent phantom text
• vulkan GPU acceleration via whisper-rs 0.16
• onboarding: interactive mic check
• system tray tooltip shows current status

Pluggable engines + multi-language

• pluggable STT engine architecture (C1)
• groq whisper cloud engine (C2)
• multi-language support (99+ languages)
• onboarding wizard with hotkey setup

Production hardening

• CLI mode (murmur-cli)
• auto-update system
• start-on-login setting

Initial public release

• local whisper.cpp transcription
• silero vad gate
• global hotkey + clipboard paste