feat: add voice coaching and hands-free voice commands during exercise execution

[DecodeX15]

PR Description:

Addressed Issues:

Fixes #(issue number) N/A

Screenshots/Recordings: N/A

Additional Notes:

Voice Coaching (TTS):

• Exercise instructions are spoken sequentially via TTS during the workout phase
• Flow: 2s delay → Instruction 1 → wait for completion → 3s gap → Instruction 2 → ...
• Instructions play as many as time allows — no instruction is cut mid-sentence
• TTS stops on pause, skip, phase change, and session finish

Hands-Free Voice Commands (STT):

• Users can control workout with voice: pause/stop/wait/hold, start/resume/play/go, skip/next
• STT runs in a continuous auto-restart loop throughout the session
• STT automatically stops while TTS is speaking (avoids speaker-to-mic audio leak) and restarts 500ms after TTS finishes
• TTS speaks a short confirmation after each command ("Paused", "Resuming", "Skipping")
• Physical buttons remain fully functional as fallback

Known Limitation:

• Voice commands are not processed while TTS is actively speaking instructions (hardware limitation — phone speaker audio leaks into mic). Commands work during gaps between instructions and after all instructions finish.

Checklist

• My code follows the project's code style and conventions
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have joined the Discord server and I will share a link to this PR with the project maintainers there
• I have read the Contributing Guidelines

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