Gate mel feature computation to reduce CPU during idle/silence

[ysdede]

Context

• Mel worker continuously computes log-mel frames on every audio chunk.
• This runs even when model is not ready or when no speech is detected.

Evidence

• src/lib/audio/mel.worker.ts: pushAudio computes FFT+mel for every frame; no gating.
• v4 path starts capture before model ready; mel still receives chunks.

Impact

• High CPU from constant feature calculation during silence/idle.

Actions

• Gate mel computation behind VAD or "model ready" state.
• Allow a lower-rate mode when only preview is needed (downsample or skip frames).
• Expose a toggle to pause mel worker when transcription is paused.

Acceptance

• CPU usage during idle recording decreases significantly.
• Trace shows fewer worker-active durations when model is not ready or silence.